Nthetic Approach Towards the Formation of Cobalt Peroxynitrite Intermediate a Dissertation
Synthetic Approach towards the Formation of Cobalt Peroxynitrite Intermediate
A dissertation submitted to the Indian Institute of Technology Guwahati as Partial fulfillment for the degree of Doctor of Philosophy in Chemistry
Submitted by Soumen Saha (Roll No. 126122002)
Supervisor Prof. Biplab Mondal
Department of Chemistry Indian Institute of Technology Guwahati December, 2017
TH-1730_126122002
Dedicated to My Family, Friends & Teachers
TH-1730_126122002
STATEMENT
I hereby declare that this thesis entitled “Synthetic Approach towards the Formation of
Cobalt Peroxynitrite Intermediate” is the outcome of research work carried out by me
under the supervision of Prof. Biplab Mondal in the Department of Chemistry, Indian
Institute of Technology Guwahati, India.
In keeping with the general practice of reporting scientific observations, due
acknowledgements have been made wherever the work described is based on the findings
of other investigators.
December, 2017 Soumen Saha
Indian Institute of Technology Guwahati
TH-1730_126122002 Acknowledgement
The success and final outcome of this thesis required a lot of guidance and assistance from many people and I am extremely privileged to have got this all along the completion of my thesis. All that I have done is only due to such supervision and assistance and I would not forget to thank them.
First and foremost I would like to thank my thesis supervisor, Prof. Biplab Mondal, who fearlessly accepted me as Ph.D student under his guidance and was the main creator of the great ideas, techniques and whole background of this thesis. We experienced together all the ups and downs of routine work, the shared happiness of success and the depression of failure. It would never have been possible for me to completion of my thesis without his incredible support and encouragement. I have been extremely lucky to have a supervisor who cared so much about my work, and who responded to my questions and queries so promptly.
Besides my advisor, I would like to thank the rest of my thesis committee member Prof. B. K. Patel, Dr. Debasis Manna and Dr. A.S. Achalkumar for their encouragement, insightful advices and valuable suggestions.
I sincerely appreciate the whole hearted cooperation and valuable help rendered by the teaching and non-teaching staff of Department of Chemistry, Indian Institute of Technology Guwahati. I am also thankful to Central Instrument facility (CIF), IITG for providing instrument facility. I wish to extend my thanks to IIT Guwahati for the financial support.
My Sincere thanks to my lab senior, Somnath Da, Hemanta Da, Vikash Da, Kanhu Da, Aswini Da, Apurba Da and Pankaj Da for their support and motivation during the initial days of my stay in the lab and immense help in my research works. My heartfelt thanks to my fellow lab mates, Kuldeep, Baishakhi, Dibyajyoti Da and Rakesh for always being there and bearing with me the good and bad times during my wonderful days of Ph.D. My special words of gratitude to Kuldeep with whom I have started my Ph.D journey and also submitting our thesis at same time. His perceptive suggestions and invaluable help were quite exciting and encouraging for me to make my career in
TH-1730_126122002 research. I would also like to thank all the project students with whom I had a great opportunity to work together.
I must express my gratitude to my childhood friends Sohail and Raju for their constant help and support. My appreciation and gratefulness to my school and college friends, Sugata, Maloy, Tatha, Mojo, Pintu, DK, Arka, Rudra, Tanmay, Dinesh, Hochi, Pratik, Anirban, Ayan, Pijush, Ashraf………. for their kind help and encouragement towards the success of my life.
I thank my all the PhD colleagues at IITG- Keshab, Halder, Shilaj, Kallol, Uday, Suranjan, Hiranya, Shubhadip, Arvin, Akhtar, Sayan, Manas, Rajendra, Sahnawaz, Belal, Gourab, Sabya, Subhankar, Subhra, Soumendra…… for all the fun I have shared in last five years.
I am extremely thanked to all my teachers because of their teaching at different stages of education. Without learning from them I could not able to write this thesis.
I owe my deepest gratitude towards my better half for her eternal support and understanding of my goals and aspirations. Her infallible love and support has always been my strength. Her patience and sacrifice will remain my inspiration throughout my life. It would be ungrateful on my part if I thank Juhi in these few words.
As always it is impossible to mention everybody who had an impact to this work however, there are those whose spiritual support is even more important. I feel a deep sense of gratitude for my father, mother and sister who formed part of my vision and taught me good things that really matter in life. All the support and love they have provided me over the years was the greatest gift anyone has ever given me. I am also very much grateful to all my family members for their constant inspiration and encouragement.
Finally, I would like to thank all others who are associated with my work directly or indirectly at IIT Guwahati for their help.
Soumen Saha
Indian Institute of Technology Guwahati
TH-1730_126122002
INDIAN INSTITUTE OF TECHNOLOGY GUWAHATI North Guwahati, Assam – 781039, India
Prof. Biplab Mondal Phone : + 91-361-258-2317 Department of Chemistry Fax: + 91-361-258-2349 E-mail: [email protected]
Certificate
This is to certify that Mr. Soumen Saha has been working under my supervision since July, 2012 as
a regular Ph.D. student in the Department of Chemistry, Indian Institute of Technology Guwahati. I
am forwarding his thesis entitled “Synthetic Approach towards the Formation of Cobalt
Peroxynitrite Intermediate” being submitted for the Ph.D. degree.
I certify that he has fulfilled all the requirements according to the rules of this Institute regarding the
investigations embodied in his thesis and this work has not been submitted elsewhere for a degree.
December, 2017 Biplab Mondal
TH-1730_126122002 Contents
Page No. Synopsis i
Chapter 1: Introduction 1.1 General aspect of peroxynitrite 1 1.2 Formation of metal peroxynitrite 3 1.3 Scope of the thesis 11 1.4 References 11
Chapter 2: Nitric Oxide Reactivity of a Co(II) Complex: Reductive Nitrosylation of Co(II) followed by Release of Nitrous Oxide Abstract 16 2.1 Introduction 17 2.2 Results and discussion 18 2.3 Experimental section 25 2.4 Conclusion 28 2.5 References 28
Chapter 3: Reactivity of a Cobalt Nitrosyl Complex: Formation of a Cobalt Peroxynitrite Intermediate and Transfer of the Nitrosyl to a Cobalt Porphyrin Complex Abstract 33 3.1 Introduction 34 3.2 Results and discussion 35 3.3 Experimental section 41 3.4 Conclusion 46 3.5 References 46
Chapter 4: Reaction of a Nitrosyl Complex of Cobalt Porphyrin with Hydrogen Peroxide: Putative Formation of Peroxynitrite Intermediate Abstract 50 4.1 Introduction 51 4.2 Results and discussion 52 4.3 Experimental section 58
TH-1730_126122002 4.4 Conclusion 61 4.5 References 61
Chapter 5: Reaction of a Co(III)-Peroxo Complex and NO: Formation of a Putative Peroxynitrite Intermediate Abstract 66 5.1 Introduction 67 5.2 Results and discussion 68 5.3 Experimental section 73 5.4 Conclusion 76 5.5 References 77
Appendix I 81 Appendix II 87 Appendix III 97 Appendix IV 104 List of publications 109
TH-1730_126122002 Synopsis
The thesis entitled “Synthetic Approach towards the Formation of Cobalt
Peroxynitrite Intermediate” is divided into five chapters.
Chapter 1: Introduction
Nitric oxide (NO) is an important small molecule known as ubiquitous intercellular
messenger in all vertebrates.1-4 Only a submicromolar concentration of NO is responsible
for all of its activities. It is known as a cytotoxic effector when produced in excess.4
Cytotoxicity of NO is due to the formation of secondary reactive nitrogen species (RNS)
- like peroxynitrite (ONOO ) or nitrogen dioxide (NO2). The formation of these secondary
RNS may result from the oxidation of NO in the presence of oxidants like superoxide
.- 5-6 radicals (O2 ), hydrogen peroxide (H2O2) and/or transition metal ions. Peroxynitrite can
- be generated by the reaction of H2O2 and nitrite (NO2 ) in the presence of the peroxidase
enzymes.7-8 On the other hand, the oxy-heme {i.e. iron(III)-superoxo} species of the nitric
- oxide dioxygenase (NODs) react with NO to result in nitrate (NO3 ) ion which is
biologically benign.9 This reaction is believed to proceed through the formation of ONOO-
intermediate.9-10 In literature, ONOO- intermediates are exemplified to form either in the
III _ - reaction of oxy-heme (formally, Fe O2 ) proteins with NO or metal-nitrosyls with O2 or
- 10-11 O2 . Clarkson and Basolo reported the reaction of a cobalt-nitrosyl complex with O2 to
- - 12 result in the formation of NO2 product through a putative ONOO intermediate. Kim et.
- 13 al. reported the reaction of a non-heme dinitrosyliron with O2 to afford NO3 . In a similar
- - reaction, [Cu(NO)] species was found to result in corresponding NO2 and O2 via ONOO
intermediate.14 Recently, the ONOO- intermediates have been shown to from in the
reaction of NO with Cr–superoxo or peroxo species. A CrIV-peroxo complex, [(12-
+ TMC)Cr(O2)(Cl)] {12-TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane} is
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Synopsis
reported to react with NO to result in a Cr(III)-nitrato complex via the formation of a
Cr(III)-(ONOO-) intermediate.15 Nam et. al. reported the reactivity of an Fe(III)-peroxo
+ + - complex, [(14-TMC)Fe(O2)] with NO as an approach for generating the ONOO
intermediate which led to the formation of an Fe(III)-nitrato complex.16
- In our laboratory, we have been working on the reactivity of the metal nitrosyls with O2
- 10 anion and H2O2 with an aim to generate a ONOO species. For example, {CuNO}
complex of the bis(2-ethyl-4-methylimidazol-5-yl)methane ligand was found to react with
H2O2 to afford the corresponding Cu(I)-nitrite and the formation of a Cu(I)-peroxynitrite
17 - intermediate is presumed. The same nitrosyl complex, upon reaction with O2 anion was
observed to result in Cu(II)-nitrite.18 In an another example, the {CuNO}10 complex in the
- 18 presence of H2O2 was shown to form Cu(II)-nitrite via a presumed ONOO intermediate.
Recently Nam group prepared a [(12-TMC)Co(NO)]2+ by transferring the bound NO
ligand from [(14-TMC)Co(NO)]2+. In addition to the NO-transfer reaction, dioxygenation
2+ + of [(14-TMC)Co(NO)] by O2 produces [(14-TMC)Co(NO3)] via the formation of a
Co(II)-(ONOO-) intermediate.19
Metal-peroxynitrite complexes decompose readily to form either metal nitrate or metal-oxo
- complex and NO2. In both cases ONOO was found to induce phenol ring nitration which
may have consequences for the process of tyrosine phosphorylation. The presence of
nitrated tyrosine residues has been suggested as a marker for peroxynitrite activity.20-21
•- Karlin et. al. reported that NO reacts with [(thf)(F8)Fe(O2 )], followed by the addition of
the tyrosine mimic 2,4-di-tert-butylphenol (DTBP) resulted in the formation of
- 22 [(F8)Fe(NO3 )] and effective nitration of DTBP occurs.
The present thesis is originated from our interest to study the formation of cobalt
- peroxynitrite intermediates by the reaction of a cobalt-nitrosyl with O2 / H2O2 or a cobalt-
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peroxo complex with NO. The second chapter describes the reaction of a Co(II) complex
with NO and the formation of a Co-nitrite complex via a thermally unstable Co-dinitrosyl
complex with the release of N2O. The third chapter discusses the reactivity of a nitrosyl
complex of cobalt salen complex. The complex was found to react with KO2 to result in
the formation of a Co-nitrate complex via a putative Co-(ONOO-) intermediate. In
addition, the bound NO group of the nitrosyl complex was transferred to a cobalt
porphyrinate to form the corresponding nitrosyl. The fourth chapter deals with the reaction
of a nitrosyl complex of cobalt porphyrinate with H2O2 to result in the cobalt nitrite
complex through a putative formation of Co-(ONOO-) intermediate. The last chapter
describes the formation of a Co-(ONOO-) intermediate in the reaction of a cobalt-peroxo
complex with NO leading to the formation of a cobalt-nitrate complex.
Chapter 2: Nitric Oxide Reactivity of a Co(II) Complex: Reductive Nitrosylation of
Co(II) Complex Followed by the Release of N2O
A Co(II) complex, [(L1)2Co]Cl2, 2.1 was prepared by stirring a mixture of cobalt(II)
chloride hexahydrate with 2 equivalents of the ligand L1 {L1 = bis(2-ethyl-4-
methylimidazol-5-yl)methane} in methanol. It was characterized structurally as well as by
spectral analyses. The ORTEP diagram is shown in figure S1a. The crystal structure
revealed that the Co(II) center is surrounded by the four nitrogen atoms from two units of
the ligand in an overall distorted tetrahedral geometry.
Gradual addition of NO gas to a dry and degassed methanol solution of 2.1 resulted in the
change of color to red. The complete reaction was observed with 3 equivalents of NO. In
UV-visible spectroscopic monitoring, the addition of NO resulted in the appearance of
absorption bands at 516 nm (ε/M-1cm-1, 300), 556 nm (ε/M-1cm-1, 490), 579 nm (ε/M-1cm-1,
474), 664 nm (ε/M-1cm-1, 140) along with other intra-ligand transitions at lower
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wavelengths (Figure S2a). This is attributed to the formation of intermediate complex 2.2.
In absence of air and moisture, the color was found to be stable for several hours.
(a) (b)
Figure S1. ORTEP diagram of complexes (a) 2.1 and (b) 2.3. (30% thermal ellipsoid plot. H-atoms and solvent molecules are not shown for clarity).
Unfortunately we could not isolate it as solid owing to its reactive nature. However,
10 spectroscopic characterization revealed 2.2 as a Co(I)-dinitrosyl having {Co(NO)2}
configuration.
(a) (b) Figure S2. (a) UV-visible spectral monitoring of the reaction of complex 2.1 (black) with NO to result, 2.2 (red) and 2.3 (green) in methanol. (b) Solution FT-IR spectral monitoring of the reaction of complex 2.1 (black) with NO to result 2.2 (red) and 2.3 (blue) in methanol.
The addition of successive equivalent of NO resulted in the change of color to dark brown.
In the visible region of the spectrum it shows absorption bands at 523 nm (ε/M-1cm-1, 184),
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558 nm (ε/M-1cm-1, 312) and 579 nm (ε/M-1cm-1, 360), respectively. This is attributed to
the formation of complex, [(L1)2Co(NO2)], 2.3 (Scheme S1). In the FT-IR spectrum, 2.2
shows two new stretching frequency at 1857 and 1774 cm-1, assignable to the coordinated
10 NO stretching of {Co(NO)2} moiety (Figure S2b). In the presence of further equivalent
of NO, the NO stretching frequencies at 1857 and 1774 cm-1 disappeared with the
appearance of a new one at 1261 cm-1 (Figure S2b). This is attributed to the coordinated
- NO2 stretching and this assignment is in well agreement with the formulation of 2.3.
In the X-band EPR spectroscopy, the characteristic signals of Co(II) in tetrahedral
geometry disappeared upon addition of 3 equivalents of NO (Figure S3a). However, the
addition of subsequent equivalent of NO resulted in the appearance of Co(II) signals again.
This suggests the formation of a diamagnetic intermediate, 2.2, upon addition of 3
equivalent of NO to the solution of 2.1. The intermediate resulted to a paramagnetic
complex, 2.3 in presence of further equivalent of NO (Figure S3a).
(a) (b) Figure S3. (a) X-band EPR spectral monitoring of the reaction of complex 2.1 (black) with NO to result, 2.2 (red) and 2.3 (green) in methanol at 77 K. (b) ESI-mass spectrum of complex 2.2 in methanol. Inset: (i) experimental and (ii) simulated isotopic distribution pattern.
1 10 H-NMR studies also suggested the formation of complex 2.2 via a {Co(NO)2}
intermediate (Figure S4). Finally, the ESI-mass spectrum also confirmed the formation of
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Co(I)-dinitrosyl complex, [{(L1)Co(NO)2} = 351.02 (molecular ion peak)] in the reaction
(Figure S3b and Scheme S1). Further, the formation of 2.3 in the reaction was confirmed
by its isolation and structural characterization. The ORTEP diagram of 2.3 is shown in
figure S1b.
Figure S4. 1H-NMR spectrum of (a) ligand L1 (black), (b) complex 2.1 (violet), (c) after addition
of 3 equivalent of NO, 2.2+L1 and (d) after addition of excess NO, complex 2.3 in CD3OD. The * marked signals are for complex 2.2.
Thus, a total of 4 equivalents of NO is required to result in the formation 2.3 from 2.1 via
the intermediate formation of 2.2 (Scheme S1). The reaction of 2.1 with NO can be
envisaged as the reductive elimination in the first step leading to the formation of 2.2,
+ [(L1)Co(NO)2] (Scheme S1). In this case one equivalent of NO is required for the
reduction of Co(II) to Co(I) and other 2 equivalents of NO form the dinitrosyl complex in
subsequent step. The NO induced reduction of Co(II) was confirmed by the presence of
MeONO in the reaction mixture in GC-mass analyses. In the next step, the dinitrosyl
complex reacts with equivalent amount of NO to result in the nitrito complex, 2.3 (Scheme
- S1). It is logical to believe that the formation of NO2 occurs by the attack of subsequent
10 equivalent of the NO to the {Co(NO)2} intermediate with concomitant release of N2O.
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Scheme S1. Reaction of complex 2.1 with NO.
Chapter 3: Reactivity of a Cobalt Nitrosyl Complex: Formation of a Peroxynitrite
Intermediate and Transfer of the Nitrosyl to a Cobalt Porphyrin Complex
In this chapter the NOD reactivity of a nitrosyl of a cobalt salen complex was explored.
The Co(II) complex 3.1, [(L2)Co] {L2 = 6,6'-((1E,1'E)-(ethane-
1,2diyl)bis(azanylylidene))bis(methanylylidene)bis(2,4-di-tert-butylphenol)} have been
synthesized and characterized structurally. The ORTEP diagram is shown in figure S5a.
Addition of NO to the dry and degassed THF solution of the 3.1 resulted in the formation
of complex 3.2. It was isolated and characterized by spectral analyses as well as by single
crystal X-ray structure determination. The ORTEP diagram is shown in figure S5b. In FT-
IR spectrum complex 3.2 displayed the coordinated nitrosyl stretching frequency at 1650
cm-1. Structural characterization revealed the presence of bent nitrosyl in the complex 3.2.
The reactivity of complex 3.2 towards O2 and KO2 has been explored. It was observed that
complex 3.2 was inert towards O2.
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However, when complex 3.2 in THF solution was subjected to react with KO2 at -40 °C, it
resulted in the formation of corresponding ONOO- intermediate. Thermal instability and
reactive nature precluded its isolation and spectral characterization.
(a) (b)
Figure S5. ORTEP diagrams of complexes (a) 3.1 and (b) 3.2. (30% thermal ellipsoid plot. H- atoms and solvent molecules are not shown for clarity).
The formation was supported by the characteristic ring nitration reaction of 2,4-di-tert-
butylphenol to result in 2,4-di-tert-butyl-6-nitrophenol (~60%) (Scheme S2).
Scheme S2. Formation of ONOO- intermediate in THF at -40 °C.
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- Further, isolation of corresponding NO3 complex, 3.3 as the isomerization product
suggests the formation of the ONOO- intermediate (Scheme S2). Complex 3.3 was
characterized by elemental analyses and spectroscopic techniques. Even after several
attempts, X-ray quality crystals were not obtained.
On the other hand, the NO transfer from complex 3.2 to 3.4, [(L3)Co] {L3 = 5,10,15,20-
tetrakis(4'-bromophenyl)porphyrinate dianion} was studied (Scheme S3). When equivalent
amount of 3.2 was subjected to mix with 3.4 in dry and degassed THF under Ar
atmosphere, a dark red solution was formed. In UV-visible spectroscopy, the 412 nm (ε/
M-1cm-1, 2.01 × 105) and 527 nm (ε/M-1cm-1, 2.8 × 104) bands of 3.4 were found to shift
416 nm (ε/M-1cm-1, 1.85 × 105) and 540 nm (ε/M-1cm-1, 2.6 × 104), respectively (Figure
S6a).
In solution FT-IR spectroscopy the appearance of a new stretching at 1720 cm-1 suggests
the formation of complex [(L3)Co(NO)], 3.5 (Figure S6b). It was isolated as solid and
characterized via spectral analyses and structure determination. The ORTEP diagram is
shown in figure S7b.
(a) (b)
Figure S6. (a) UV-visible spectral monitoring of the reaction of complex 3.2 with 3.4 (black) to result 3.5 (red) in THF. (b) Solution FT-IR monitoring of the reaction of complex 3.2 (black) with 3.4 to result 3.5 (blue) in THF.
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(a) (b)
Figure S7. ORTEP diagram of complexes (a) 3.4 and (b) 3.5. (30% thermal ellipsoid plot. H-atoms are not shown for clarity).
Scheme S3. NO transfer from complex 3.2 to 3.4.
Chapter 4: Reaction of a Nitrosyl Complex of Cobalt Porphyrin with Hydrogen
Peroxide: Putative Formation of Peroxynitrite Intermediate
A Co(II)-porphyrinate complex, [(L4)Co], 4.1, {L4 = 5,10,15,20-tetrakis(4'chlorophenyl)
porphyrinate dianion} resulted in the corresponding nitrosyl complex, [(L4)Co(NO)], 4.2,
upon reaction with NO in dichloromethane (Scheme S4). The complex 4.2 was isolated as
solid and characterized spectroscopically as well as structurally (Figure S8). In the UV-
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visible spectrum, 4.2 exhibited characteristic absorption bands at 415 nm (ε/M-1cm-1,
2.21×105) and 538 nm (ε/M-1 cm-1, 3.1×104) (Figure S9a).
Scheme S4. Synthesis of nitrosyl complex 4.2.
Complex 4.2 was silent in the X-band EPR spectroscopy suggesting the presence of
Co(III) centre having [CoIII-NO-] or {CoNO}8 description. In the FT-IR spectrum, the NO
stretching appeared at 1701 cm-1 (Figure S9b).
Figure S8. ORTEP diagram of complex 4.2 (30% thermal ellipsoid plot. H-atoms and solvent molecules are not shown for clarity).
(a) (b)
Figure S9. (a) UV-visible spectra of complexes 4.1 (black) and 4.2 (red) in dichloromethane. (b) FT-IR spectrum of complex 4.2 in KBr.
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It was observed that complex 4.2 in CH2Cl2 solution is inert toward O2. However, in
CH2Cl2/CH3CN solution it was found to react with H2O2. Addition of H2O2 to the
CH2Cl2/CH3CN solution of 4.2 at -40 C resulted in the corresponding Co-nitrito complex,
4.3 (Scheme S5).
Scheme S5. Reaction of 4.2 with H2O2 in dichloromethane/acetonitrile.
In the UV-visible spectrum, the characteristic absorption bands of 4.2 at 415 and 538 nm
disappeared with the formation of new ones at 430 nm (ε/M-1cm-1, 2.84×105), 546 (ε/M-1
cm-1, 3.3×104) and 665 nm (ε/M-1cm-1, 4.2×103), respectively (Figure S10). Spectral
titration suggested that the stoichiometric ratio of complex 4.2 with H2O2 is 1:1. Complex
4.3 was isolated and characterized as the corresponding CoIII-nitrito complex.
Figure S10. UV-visible spectral monitoring of the reaction of complex 4.2 (black) with H2O2 to result in 4.3 (red) at -40 C.
The ESI mass spectrum of 4.3 displayed the molecular ion peak (M+1) at m/z, 854.99
(calcd. 856.08). In the FT-IR spectrum, the stretching frequency at 1270 cm-1 is assignable
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- to the NO2 stretching. The formation of cobalt-nitrito complex is expected to be
accompanied by the release of O2 (Scheme S5). The formation of O2 in the reaction was
confirmed by its detection in the headspace gas of the reaction flask using alkaline
pyrogallol test. While we do not have direct spectroscopic evidence of formation of Co-
(ONOO-) intermediate, we sought chemical evidence for the postulated species. Effective
nitration (ca. 50%) and corresponding Co-hydroxo product, 4.4 (ca.70%) were observed
when DTBP was added to complex 4.2 before the addition of H2O2 (Scheme S6).
Scheme S6. Phenol ring nitration by 4.2 in the presence of H2O2.
Chapter 5: Reaction of a Co(III)-Peroxo Complex and NO: Formation of a Putative
Peroxynitrite Intermediate
The addition of H2O2 in presence of NEt3 to the methanol solution of complex 2.1 at
+ -40 °C afforded Co(III)-peroxo intermediate, [(L1)2Co(O)2] , 5.1 (Scheme S7). It
displayed absorption at 452 nm (ε/M-1cm-1, 890) in the UV-visible spectrum (Figure S11a).
The O-O stretching for the Co(III)-peroxo intermediate appeared at 874 cm-1 in FT-IR
spectrum (Figure S11b). As expected for a Co(III)-peroxo species, the intermediate was
silent in X-band EPR study (Figure S12a). The ESI-mass spectrum of the intermediate was
18 18 sensitive towards O-labelling and found to appear at m/z 559.41 when H2 O2 was used
(Figure S12b). The addition of NO to the freshly generated complex 5.1 in methanol at -
40 °C followed by warming at room temperature resulted in the formation of complex,
[(L1)2Co(NO3)]Cl, 5.2. The ORTEP diagram of 5.2 is shown in figure S13.
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Scheme S7. Reaction of complex 2.1 with H2O2 in methanol at -40 C.
(a) (b)
Figure S11. (a) UV-visible spectra of complex 2.1 (red), after addition of H2O2 {complex 5.1} (grey), in methanol at -40 °C. (b) Solution FT-IR spectra of complexes 2.1 (black), 5.1 (red) in methanol. Green and blue traces represent the gradual decomposition of 5.1 at room temperature.
(a) (b)
Figure S12. (a) X-band EPR spectra of complexes 2.1 (black) and 5.1 (red) in methanol at 77 K. 16 (b) ESI-Mass spectra of complex 5.1 obtained from the reaction of 2.1 and H2 O2 in methanol. 18 Inset shows the same when the reaction was carried out with H2 O2.
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The crystal structure revealed that the Co(II) center is bonded with two units of the ligand
- and a nitrate (NO3 ) anion in a distorted octahedral geometry (Figure S13 ).
Figure S13. ORTEP diagram of complex 5.2 (30% thermal ellipsoid plot. H-atoms and solvent molecule are not shown for clarity).
The formation of 5.2 can be envisaged through the formation of a putative ONOO-
intermediate (5.1a) in the course of the reaction (Scheme S8). When DTBP was added
before the addition of NO to the solution of 5.1, an appreciable nitration (ca. 60%) was
observed. Work-up of the reaction mixture revealed the formation of the corresponding
Co-hydroxo product, 5.3 (ca. 70%).
Scheme S8. Formation of ONOO- intermediate in methanol at -40 C.
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(5) (a) Radi, R. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 4003. (b) Tran, N. G.;
Kalyvas, H.; Skodje, K. M.; Hayashi, T.; Moënne-Loccoz, P.; Callan, P. E.;
Shearer, J.; Kirschenbaum, L. J.; Kim, E. J. Am. Chem. Soc. 2011, 133, 1184.
(6) (a) Goldstein, S.; Lind, J.; Merenyi, G. Chem. Rev. 2005, 105, 2457. (b) Blough, N.
V.; Zafiriou, O. C. Inorg. Chem. 1985, 24, 3502. (c) Nauser, T.; Koppenol, W. H.
J. Phys. Chem. A 2002, 106, 4084. (d) Speelman, A. L.; Lehnert, N. Acc. Chem.
Res. 2014, 47, 1106. (e) Fry, N. L.; Mascharak, P. K. Acc. Chem. Res. 2011, 44,
289.
(7) Qiao, L; Lu, Y.; Liu, B.; Girault, H. H. J. Am. Chem. Soc. 2011, 133, 19823.
xvi
TH-1730_126122002
Synopsis
(8) Vilet, A.; Eiserich, J. P.; Halliwell, B.; Cross. C. E. J. Biol. Chem. 1997, 272,
7617.
(9) (a) Pfeiffer, S.; Gorren, A. C. F.; Schmidt, K.; Werner, E. R.; Hansert, B.; Bohle,
D. S.; Mayer, B. J. Biol. Chem. 1997, 272, 3465. (b) Coddington, J. W.; Hurst, J.
K.; Lymar, S. V. J. Am. Chem. Soc. 1999, 121, 2438. (c) Koppenol, W. H.; Bounds,
P. L.; Nauser, T.; Kissner, R.; Rüegger, H. Dalton Trans. 2012, 41, 13779.
(10) (a) Schopfer, M. P.; Wang, J.; Karlin, K. D. Inorg. Chem. 2010, 49, 6267. (b)
Ouellet, H.; Ouellet, Y.; Richard, C.; Labarre, M.; Wittenberg, B.; Wittenberg, J.;
Guertin, M. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 5902. (c) Gardner, P. R.;
Gardner, A. M.; Martin, L. A.; Salzman, A. L. Proc. Natl. Acad. Sci. U. S. A. 1998,
95, 10378. (d) Ford, P. C.; Lorkovic, I. M. Chem. Rev. 2002, 102, 993. (e) Gardner,
P. R.; Gardner, A. M.; Brashear, W. T.; Suzuki, T.; Hvitved, A. N.; Setchell, K. D.
R.; Olson, J. S. J. Inorg. Biochem. 2006, 100, 542.
(11) Kurtikyan, T. S.; Ford, P. C. Chem. Commun. 2010, 46, 8570.
(12) (a) Clarkson, S. G.; Basolo F. J. Chem. Soc. Chem. Commun. 1972, 119, 670. (b)
Clarkson, S. G.; Basolo F. Inorg. Chem. 1973, 12, 1528.
(13) Skodje, K. M.; Williard, P. G.; Kim, E. Dalton Trans. 2012, 41, 7849.
(14) Park, G. Y.; Deepalatha, S.; Puiu, S. C.; Lee, D.-H.; Mondal, B.; Sarjeant, A. A. N.;
Del Rio, D.; Pau, M. Y. M.; Solomon, E. I.; Karlin, K. D. J. Biol. Inorg. Chem.
2009, 14, 1301.
(15) (a) Yokoyama, A.; Han, J. E.; Cho, J.; Kubo, M.; Ogura, T.; Siegler, M. A.; Karlin,
K. D.; Nam, W. J. Am. Chem. Soc. 2012, 134, 15269. (b) Yokoyama, A.; Cho, K.
B.; Karlin, K. D.; Nam, W. J. Am. Chem. Soc. 2013, 135, 14900.
(16) Yokoyama, A.; Han, J. E.; Karlin, K. D.; Nam, W. Chem. Commun. 2014, 50,
1742.
xvii
TH-1730_126122002
Synopsis
(17) Kalita, A.; Kumar, P.; Mondal, B. Chem. Commun. 2012, 48, 4636.
(18) Kalita, A.; Deka, R. C.; Mondal, B. Inorg. Chem. 2013, 52, 10897.
(19) Kumar, P.; Lee, Y.-M.; Hu, L.; Chen, J.; Park, Y. J.; Yao, J.; Chen, H.; Karlin, K.
D.; Nam, W. J. Am. Chem. Soc. 2016, 138, 7753.
(20) Mondoro, T. H.; Shafer, B. C.; Vostal, J. G. Free Radical Biol. Med. 1997, 22,
1055.
(21) Kong, S. K.; Yim, M. B; Stadtman, E. R.; Chock, P. B. Proc. Natl. Acad. Sci. U. S.
A. 1996, 93, 3377.
(22) Schopfer, M. P.; Mondal, B.; Lee, D.-H.; Sarjeant, A. A. N.; Karlin, K. D. J. Am.
Chem. Soc. 2009, 131, 11304.
xviii
TH-1730_126122002 Chapter 1
Introduction
1.1 General aspect of peroxynitrite
Nitric oxide (NO) is an important small molecule known as ubiquitous intercellular
messenger in all vertebrates, modulating blood flow, thrombosis, and many more neural
activities. Only a small concentration of NO is required for its function.1-3 However, over
- production of it has a detrimental effect due to the reaction of NO with superoxide (O2 ) to
form the peroxynitrite (ONOO-, PN).4 Under physiological conditions, the concentration of
- O2 is regulated by superoxide dismutase (SOD) enzymes and NO is present in pM to nM
- concentrations. Enhanced release of NO results in the reaction with O2 at a diffusion-
limited rate to form ONOO-.5,6 Alternatively, ONOO- can be generated by the reaction of
- 7,8 hydrogen peroxide (H2O2) and nitrite (NO2 ) in the presence of the peroxidase enzymes.
Peroxynitrite induces oxidation and nitrosation of biomolecules. It is also known to play
roles in diverse chemical modifications to lipids, proteins and nucleic acids.5,9,10 For
example, Beckman et al. showed ONOO- as a far more effective means of producing
hydroxyl radical than the widely accepted reaction of reduced iron with H2O2 (known as
the Fenton reaction or the iron-catalyzed Haber-Weiss reaction).11 Peroxynitrite, being a
strong oxidant can react with electron-rich groups, such as sulfhydryls12, iron-sulfur
centers13, zinc-thiolates14 and the active site of sulfhydryl in tyrosine phosphatases.15-17 At
- near neutral pH, ONOO rapidly decomposes (t1/2 <1s) to peroxynitrous acid (ONOOH),
- which has pKa of 6.75 at 37 °C. It is unstable and rapidly isomerizes to nitrate (NO3 )
(Eq. 1).18
……… (Eq. 1)
TH-1730_126122002
Chapter 1
ONOOH is strong oxidant in reactions of both one and two electron mechanisms (Eq. 2 &
3). In the presence of biological targets these reactions lead to a range of oxidized
- products. ONOO also reacts rapidly with CO2 to form the nitroso peroxycarbonate
anion.19
………. (Eq. 2)
………... (Eq. 3)
The primary effect of ONOO- on proteins is the nitration of the tyrosine residues.20,21
Nitrated tyrosine residues in peptides cannot be phosphorylated by tyrosine kinases in
vitro. Thus, nitration of proteins can prevent or modulate cellular signal transduction,
which requires tyrosine phosphorylation.22,23 The transition metal ions have been found to
catalyse the reaction of ONOO-, in either one or two electron redox processes.24 Nitric
Oxide Deoxygenases enzymes (NODs) is known to control the over production of NO in
- vivo by converting it to the biologically benign NO3 . This reaction is also believed to
proceed through a ONOO- intermediate.4
Metal-peroxynitrite complexes decompose readily to form either metal nitrate
- (isomerisation product) or metal-oxo complex and NO2 (Eq. 4). In both cases ONOO
appears to bring about tyrosine nitration. The presence of nitrated tyrosine residues has
- been suggested as a marker for ONOO activity. The formation of NO2 has been proposed
- as an essential step in tyrosine nitration by ONOO . The NO2 itself could attack tyrosine as
a two-electron acceptor to yield a nitroarenium ion intermediate and gives a nitration
product (Eq. 5).25,26
n · · n · n+1 ONOO-M L → NO2 + O-M L → NO2 + O=M L …… (Eq. 4)
n n n + NO2-O-M L + TyrH → [TyrH-NO2-O-M L] → NO2-Tyr + O=M L + H ……….(Eq. 5)
2
TH-1730_126122002
Chapter 1
1.2 Formation of metal peroxynitrite
1.2.1 Reaction of metal nitrosyls with reactive oxygen species
Clarkson and Basolo reported the involvement of a few cobalt-nitrosyl complexes with O2
in the formation of corresponding cobalt-peroxynitrite intermediate which decomposes to
yield corresponding cobalt nitrite product.27 Karlin group have reported that the reaction of
NO with a Cu(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)-(NO)
- adduct, which when exposed to O2 generates a Cu(II)-(ONOO ) species. This undergoes
- thermal decomposition to produce a Cu(II)-(NO2 ) complex and 0.5 mol equivalent of O2
28 (Scheme 1.1) .
- Scheme 1.1. Reaction of Cu(I)-nitrosyl complex with O2 to form Cu(II)-(ONOO ) intermediate.
10 Kim et al. reported a N-bound {Fe(NO)2} DNIC complex, [(TMEDA)Fe(NO)2]
{TMEDA = N,N,N´,N´-tetramethylethylenediamine} which reacts with O2 and acts as a
- Scheme 1.2. Reaction of Fe-DNIC with O2 to form Fe-(ONOO ) intermediate. 3
TH-1730_126122002
Chapter 1
nitrating agent that converts 2,4-di-tert-butylphenol to 2,4-di-tert-butyl-6-nitrophenol via
the formation of a ONOO- intermediate [(TMEDA)Fe(NO)(ONOO)] which is stable at -80
ºC (Scheme 1.2).29
Recently Nam group have reported two Co(III)-nitrosyl complexes bearing N-
tetramethylated cyclam (TMC) ligands, [(12-TMC)Co(NO)]2+ and [(13-TMC)Co(NO)]2+
- which do not react with O2; but reacts with O2 to form the corresponding Co(II)-nitrito
- 30 complexes and O2 via a presumed Co(II)-(ONOO ) intermediate (Scheme 1.3).
- - Scheme 1.3. Reaction of Co(III)-nitrosyl with O2 to from Co(II)-(ONOO ) intermediate.
Whereas the Co(III)-nitrosyl complex bearing 14-TMC ligand, [(14-TMC)Co(NO)]2+ on
+ dioxygenation produces [(14-TMC)Co(NO3)] . The reaction is proposed to occur via the
formation of a putative Co(II)-(ONOO-) intermediate (Scheme 1.4).31
- Scheme 1.4. Reaction of Co(III)-nitrosyl with O2 to form Co(II)-(ONOO ) intermediate. 4
TH-1730_126122002
Chapter 1
Our laboratory has been actively involved in studying the formation and decomposition
pathways of metal peroxynitrite. In this regard, a Cu(II)-nitrosyl complex with histidine-
derived ligand, methyl 2-(2-hydroxybenzylamino)-3-(1H-imidazol-5-yl)propanoate) have
32 been reported. In acetonitrile medium, the presence of H2O2 resulted in the formation of
the corresponding Cu(I)-(ONOO-) (Scheme 1.5).
- Scheme 1.5. Reaction of Cu(II)-nitrosyl with H2O2 to form Cu(I)-(ONOO ) intermediate.
The formation of ONOO- species is confirmed by characteristic phenol ring nitration
reaction which resembles the tyrosine nitration in biological systems. Further, isolation of
- the nitrate (NO3 ) as the decomposition product from nitrosyl complex at room temperature
also supports the involvement of ONOO- (Scheme 1.5).
Another Cu(II)-nitrosyl complex with bis(2-ethyl-4-methyl-imidazol-5-yl)methane ligand
has been found to react with H2O2 at -20 °C in acetonitrile to result in the formation of the
Cu(I)-nitrite via putative formation of the corresponding Cu(I)-peroxynitrite intermediate.
The formation of the ONOO- intermediate also confirmed by its characteristic phenol ring
nitration (Scheme 1.6).33
5
TH-1730_126122002
Chapter 1
- Scheme 1.6. Reaction of Cu(II)-nitrosyl with H2O2 to form Cu(I)-(ONOO ).
1.2.2 Reaction of metal-oxygenated species with NO
Alternatively, there are examples in literatures which demonstrate the in situ generation of
ONOO- in the reaction of reactive metal-dioxygen complexes with NO. Hemoglobin (Hb)
34 III - and myoglobin (Mb) exhibit NODs activity. NO reacts with oxyHb [Hb(Fe -O2 )],
III - III - oxyMb [Mb(Fe -O2 )] and oxyflavohemoglobin [flavoHb(Fe -O2 )] and gives nitrate
product.35 The generally stated mechanism of NODs involves direct reaction of the Fe(III)-
- - (O2 ) oxy complex with NO, giving a ONOO intermediate. Subsequent homolytic O-O
IV bond cleavage produces an oxo-ferryl (Fe =O) species and NO2; the latter attacks the
ferryl O-atom to produce a N-O bond giving nitrate (Scheme 1.7).36
Scheme 1.7. Reaction of Fe(III)-superoxo with NO to form Fe(III)-(ONOO-) intermediate.
6
TH-1730_126122002
Chapter 1
III - Karlin et al. reported that NO reacts with [(thf)(F8)Fe -(O2 )] which can lead to the
III - 37 formation of [(F8)Fe -(NO3 )]. Effective nitration was observed when DTBP (≥1 equiv)
was added prior to the addition of NO (Scheme 1.8).
Scheme 1.8. Reaction of Fe(III)-superoxo with NO to form Fe(III)-(ONOO-) intermediate.
Nam group describes the reaction of a (14-TMC)Fe(III)-peroxo complex with NO+ to
produce an (14-TMC)Fe(IV)-oxo complex and NO2 via the formation of a putative Fe(III)-
(ONOO-) species which undergoes O–O bond homolysis. Finally the reaction mixture
leads to the formation of a (14-TMC)Fe(III)-nitrate complex (Scheme 1.9).38
Scheme 1.9. Reaction of Fe(III)-peroxo with NO to form Fe(III)-(ONOO-) intermediate.
7
TH-1730_126122002
Chapter 1
The reaction of a Cu(II)-hydroperoxo species with NO resulted in the generation of
- + ONOO . The copper-hydroperoxo complex, [(BA)Cu(OOH)] (BA, a tetradentate N4
ligand possessing a pendant -N(H)CH2C6H5 group), reacts with excess NO in acetone at -
- 39 90 °C to form a Cu(II)-nitrate via an intermediacy of a ONOO species (Scheme 1.10).
Scheme 1.10. Reaction of Cu(II)-hydroperoxo with NO to form Cu(I)-(ONOO-) intermediate.
- The same group reported a discrete Cu(II)-(ONOO ) complex, [(TMG3tren)Cu
+ (OONO)] (TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine)
Scheme 1.11. Reaction of Cu(II)-superoxo with NO to form Cu(II)-(ONOO-).
8
TH-1730_126122002
Chapter 1
- + from the reaction of NO with a Cu(II)-O2 adduct, [(TMG3tren)Cu(O2 )] which finally
+ 40 resulted in a nitrite complex, [(TMG3tren)Cu(ONO)] and O2 (Scheme 1.11).
Kurtikyan group reported an oxy-coboglobin model compounds (Py)Co(Por)(O2) [por =
meso-tetraphenyl- and meso-tetra-p-tolylporphyrinato dianions], which react with NO at
1 low temperature to form a six coordinated nitrato complex (Py)Co(Por)(η -ONO2) via
initial formation of a six-coordinate ONOO- intermediate (Py)Co(Por)(η1-OONO).41 The
1 nitrato complex, (Py)Co(Por)(η -ONO2), is also not thermally stable and decomposes to
- give NO3 and oxidized cationic complex, (Py)2Co(Por). However, in the presence of
excess NO, the nitro−pyridine complexes (Py)Co(Por)(NO2) are predominantly formed,
1 indicating the oxo-transfer reactivity of (Py)Co(Por)(η -ONO2) with regard to NO.
+ A stable peroxo complex, [(12-TMC)Cr(O2)(Cl)] was reported to react with NO to form a
Cr-nitrato complex via the formation of a Cr(III)-(ONOO-) intermediate. A Cr(IV)-oxo
- species and NO2 radical was formed during the decomposition of ONOO intermediate
(scheme 1.12).42
Scheme 1.12. Reaction of Cr(IV)-peroxo with NO to form Cr(III)-(ONOO-) intermediate.
9
TH-1730_126122002
Chapter 1
The reaction of an end-on Cr(III)-superoxo complex bearing a 14-membered
+ tetraazamacrocyclic TMC ligand, [(14-TMC)Cr(O2)(Cl)] , with NO resulted in the
generation of a stable Cr(IV)-oxo species, [(14-TMC)Cr(O)(Cl)]+, via the formation of a
Cr(III)-peroxynitrite intermediate and homolytic O-O bond cleavage of the peroxynitrite
ligand (Scheme 1.13).
Scheme 1.13. Reaction of Cr(III)-superoxo with NO to form Cr(III)-(ONOO-) intermediate. .
The formation of a Cr(III)-hydroxo complex ([(14-TMC)Cr(OH)(Cl)]+ as well as 40% of
nitrated DTBP (2,4-di-tert-butyl-6-nitrophenol, nitro-DTBP) and 12% of oxidatively
dimerized DTBP (2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butylbiphenol, DTBP dimer) product
was observed via radical mechanism (Scheme 1.14). 43,44
. OH O OH . x 2 OH HO
III Cr (OH) OH O2N (12%) IV . CrIII(ONOO-) Cr (O) + NO2
(46 %)
Scheme 1.14. Decomposition pathways of ONOO- intermediate in presence of 2,4-DTBP.
10
TH-1730_126122002
Chapter 1
1.3 Scope of the thesis
The present thesis is originated from our interest to study the formation of cobalt
- peroxynitrite intermediates by the reaction of cobalt-nitrosyls with H2O2/ O2 or a cobalt-
peroxo complex with NO. The second chapter describes the reaction of a Co(II) complex
with NO and the formation of a Co-nitrite complex via a thermally unstable Co-dinitrosyl
intermediate. The third chapter discusses the reactivity of a nitrosyl complex of cobalt with
salen ligand. The complex was found to react with KO2 to result in the formation of a Co-
nitrate complex via a putative Co-(ONOO-) intermediate. In addition, the bound NO group
of the nitrosyl complex was transferred to a cobalt porphyrinate. The fourth chapter deals
with the reaction of a nitrosyl complex of cobalt porphyrinate with H2O2 to result in the
cobalt nitrite complex through a putative formation of Co-(ONOO-) intermediate. The last
chapter describes the formation of a Co-(ONOO-) intermediate in the reaction of a cobalt-
peroxo complex with NO leading to the formation of a cobalt-nitrate complex.
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D.; Nam, W. J. Am. Chem. Soc. 2016, 138, 7753.
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(43) Yokoyama, A.; Cho, K.-B.; Karlin, K. D.; Nam, W. J. Am. Chem. Soc. 2013, 135,
14900.
(44) Cho, J.; Woo, J.; Nam W. J. Am. Chem. Soc. 2012, 134, 11112.
15
TH-1730_126122002 Chapter 2
Nitric Oxide Reactivity of a Co(II) Complex: Reductive Nitrosylation of Co(II) followed by Release of Nitrous Oxide
Abstract
A cobalt complex, [(L1)2Co]Cl2, 2.1 of the ligand L1 {L1 = bis(2-ethyl-4-methylimidazol-
5-yl)methane} has been synthesized and characterized structurally. Addition of 3
equivalents of NO to a dry and degassed methanol solution of 2.1, resulted a Co-dinitrosyl
complex, [(L1)Co(NO)2]Cl, 2.2 via the reductive nitrosylation pathway. The addition of
- further equivalent of NO to the reaction mixture afforded the nitrite (NO2 ) complex,
[(L1)2Co(NO2)]Cl, 2.3 with simultaneous release of N2O. Spectroscopic analyses and
isotope labelling experiments confirm the formation of complex 2.2 as an intermediate,
whereas 2.3 characterized structurally.
TH-1730_126122002
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2.1 Introduction
Nitric oxide (NO) plays diverse roles in biological processes such as neurotransmission,
immune response, regulation of blood pressure etc.1-4 Most of these reactivity are
attributed to the interaction of NO with the metallo-proteins.1-5 The biological roles of NO
inspire a wide range of studies of its coordination and interaction with transition metal
centers.6-9 NO chemistry of cobalt has not been studied as extensively as that of iron,
perhaps because of less significance of cobalt in biological systems. However, cobalt
nitrosyls are interesting for its several unique reactions. For instance, cobalt dinitrosyls
nitrosylate alkene double bonds to result in corresponding bis-nitroso compounds.10
Cobalt is also known to mediate the disproportionation of NO which is industrially
important.10 Like iron nitrosyls, a variety of cobalt nitrosyls are known; however, cobalt
mononitrosyls having {CoNO}7 and {CoNO}9 configurations are rare and only a few
examples are there in literature.11 On the other hand, recently Harrop et al. reported an
example where cobalt nitrosyl complex in a pyrrole/imine ligand framework having
{CoNO}8 configuration could serve as potential HNO donor.12 The reported {CoNO}8
complex in the presence of stoichiometric amount of H+ behaves as an HNO donor. In the
10 absence of an HNO target, the {Co(NO)2} dinitrosyl was found as the end product. Thus
the NO reactivity of cobalt complexes is a field of current research.
It has been reported recently that the ligand denticity and geometry play a considerable
role in controlling the reactivity of NO with transition metal complexes. For instance,
Cu(II) complex of macrocyclic 5,5,7,12,12,14-hexamethyl-1,4,8,11-
tetraazacyclotetradecane ligand does not react with NO in acetonitrile solution; whereas
the same with analogous 5,5,7-trimethyl-[1,4]-diazepane ligand in acetonitrile solution
reacts with NO to afford {CuNO}10 intermediate followed by the reduction of the metal
center.13 Recently, the Lippard’s group demonstrated the influence of tetraazamacrocyclic
17
TH-1730_126122002
Chapter 2
tropocoronand ligands on the NO reactivity of their Co(II) complexes. [Co(TC-3,3)] and
[Co(TC-4,4)] and [Co(TC-5,5)] were found to result in the formation of corresponding
mononitrosyl which have been isolated and structurally characterized. On the other hand,
10 [Co(TC-6,6)] upon reaction with NO, resulted in {Co(NO)2} species. This further
14 decomposes to [Co(NO2)(TC-6,6)] complex.
The present study demonstrates the NO reactivity of a Co(II) complex, 2.1, of bis-
imidazole based ligand, L1 {L1 = bis(2-ethyl-4-methylimidazol-5-yl)methane} (Figure
2.1). The reaction resulted in the formation of Co(II)-nitrito complex, 2.3. Details
spectroscopic analyses suggest the involvement of a Co(I)-dinitrosyl complex, 2.2 having
10 {Co(NO)2} configuration. The formation of 2.2 is attributed to the reductive nitrosylation
of 2.1.
Figure 2.1. Ligand used in the present work.
2.2 Results and discussion
The ligand, L1 {L1 = bis(2-ethyl-4-methylimidazol-5-yl)methane} was synthesized by
15 following an earlier reported procedure. The Co(II) complex, [(L1)2Co]Cl2, 2.1 was
prepared by stirring a mixture of cobalt(II) chloride hexahydrate with two equivalents of
the ligand in methanol (Experimental section). The complex 2.1 was characterized by
spectroscopic analyses such as UV-visible, FT-IR, EPR spectroscopy and ESI-mass
spectrometry (Experimental section and Appendix I). The formation of the complex was
further confirmed by the single crystal X-ray structure determination. The crystallographic
data and other metric parameters listed in tables A1.1, A1.2 and A1.3 (Appendix I). The
18
TH-1730_126122002
Chapter 2
ORTEP diagram of 2.1 is shown in figure 2.2a. The crystal structure revealed that the
Co(II) center is coordinated by the four nitrogen atoms from two units of the ligand in an
overall distorted tetrahedral geometry. The presence of two chloride ions outside of the co-
ordination sphere satisfied the charge of the central metal ion. The average Co-N distance
(1.982 Å) was found to be within the range of analogous reported complexes.16
In methanol solution, 2.1 exhibited absorptions at 515 nm (ε/M-1cm-1, 323), 559 nm (ε/M-1
cm-1, 494) and 581 nm (ε/M-1cm-1, 490) along with the other intra-ligand transitions at
lower wavelengths (Figure 2.3a). These absorption bands in the visible region of the
4 4 4 4 4 4 spectrum are attributed to the A2 T1(P), A2 T1(F), A2 T2 transitions,
respectively, in tetrahedral Co(II) (d7) system.17 The crystalline 2.1 was dissolved in
methanol and electron paramagnetic resonance (EPR) was recorded at 77 K (Figure 2.4a
and Appendix I). The observed gav was 2.29 as expected for a Co(II) ion in tetrahedral
field. The observed magnetic moment was found to be 4.32 BM, which is comparable with
the other reported Co(II) tetrahedral complexes.18
(a) (b)
Figure 2.2. ORTEP diagram of complexes (a) 2.1 and (b) 2.3. (30% thermal ellipsoid plot. H- atoms and solvent molecules are not shown for clarity).
NO reactivity of complex 2.1
Gradual addition of NO gas to a dry and degassed methanol solution of 2.1 resulted in the
19
TH-1730_126122002
Chapter 2
change of color from violet to red. The complete reaction was observed with 3 equivalents
of NO. In UV-visible spectroscopic monitoring, the addition of NO resulted in the
appearance of absorption bands at 516 nm (ε/M-1cm-1, 300), 556 nm (ε/M-1cm-1, 490), 579
nm (ε/M-1cm-1, 474), 664 nm (ε/M-1cm-1, 140) along with other intra-ligand transitions at
lower wavelengths (Figure 2.3a). This is attributed to the formation of intermediate
complex 2.2. In absence of air and moisture, the color was found to be stable for several
hours. Unfortunately we could not isolate it as solid owing to its reactive nature. However,
10 spectroscopic characterization revealed 2.2 as a Co(I)-dinitrosyl having {Co(NO)2}
configuration (see later). It is to be noted that in CH2Cl2 solution
[(TMEDA)Co(NO)2](BPh4)] (TMEDA = N,N,N,´N´-tetramethylethylenediamine) absorbs
19 10 at 640 nm. Another {Co(NO)2} of mesityl derivative of ethylenediamine ligand
framework exhibits d-d transition at 600 nm.20
(a) (b) Figure 2.3. (a) UV-visible spectral monitoring of the reaction of complex 2.1 (black) with NO to result, 2.2 (red) and 2.3 (green) in methanol. (b) Solution FT-IR spectral monitoring of the reaction of complex 2.1 (black) with NO to result 2.2 (red) and 2.3 (blue) in methanol.
The addition of successive equivalent of NO resulted in the change of color to dark brown.
In the visible region of the spectrum it shows absorption bands at 523 nm (ε/M-1cm-1, 184),
558 nm (ε/M-1cm-1, 312) and 579 nm (ε/M-1cm-1, 360) nm, respectively. This is attributed
to the formation of complex, 2.3 (Scheme 2.1). In the FT-IR spectrum, 2.2 shows two new
20
TH-1730_126122002
Chapter 2
stretching frequency at 1857 and 1774 cm-1 assignable to the coordinated NO stretching of
10 {Co(NO)2} moiety (Figure 2.3b). These are found to be isotope sensitive and shifted to
-1 15 10 1825 and 1749 cm , respectively, with NO. In general, the {Co(NO)2)} complexes
show symmetric and asymmetric NO stretching frequencies between 1820-1876 cm-1 and
1750-1798 cm-1, respectively.21 The Lippard’s group have reported a number of examples
10 of Co-dinitrosyl having {Co(NO)2} configuration where the stretching frequencies appear
-1 14a,b in the range of 1700-1900 cm . For example, in [(TC-6,6)Co(NO)2], NO stretching
-1 frequency appears at 1807 and 1730 cm , whereas in case of [(TC-6,6)Co2(NO)4] it was
observed at 1799 and 1720 cm-1.14a,b It has been reported earlier that the addition of NO to
a CH2Cl2 solution of a Co(II) complex with tmeda ligand, resulted in the formation of
corresponding cobalt-dinitrosyl complex which shows NO stretching at 1866 and 1789
cm-1, respectively.19
iPr tBu Bz The reactions of [Co( DATI)2], [Co( DATI)2] and [Co( DATI)2] (DATI = dansyl
aminotroponiminates) in CH2Cl2 solution with NO were also known to form the
corresponding dinitrosyl complexes with NO stretching at 1838, 1760; 1833, 1760 and
1833, 1755 cm-1, respectively.22
In the presence of further equivalent of NO, the NO stretching frequencies at 1857 and
1774 cm-1 disappeared with the appearance of a new one at 1261 cm-1 (Figure 2.3b). This
- is attributed to the coordinated NO2 stretching and this assignment is in well agreement
- with the formulation of 2.3. In case of other reported examples, the metal coordinated NO2
stretching appears at in the range of 1260-1485 cm-1.23
In the X-band EPR spectroscopy, the characteristic signals of Co(II) in tetrahedral
geometry disappeared upon addition of 3 equivalents of NO (Figure 2.4a). However, the
addition of subsequent equivalent of NO resulted in the appearance of Co(II) signals again.
21
TH-1730_126122002
Chapter 2
The observed gav is consistent with the reported examples of Co(II) in distorted octahedral
geometry.24 This suggests the formation of a diamagnetic intermediate, 2.2, upon addition
of 3 equivalents of NO to the solution of 2.1. The intermediate resulted to a paramagnetic
complex, 2.3 in presence of further equivalent of NO (Figure 2.4a).
(a) (b) Figure 2.4. (a) X-band EPR spectral monitoring of the reaction of complex 2.1 (black) with NO to result, 2.2 (red) and 2.3 (green) in methanol at 77 K. (b) ESI-mass spectrum of complex 2.2 in methanol. Inset: (i) experimental and (ii) simulated isotopic distribution pattern.
1 In H-NMR spectrum, the broad peaks of paramagnetic 2.1 in CD3OD became well
resolved in the presence of 3 equivalents of NO (Figure 2.5). The spectrum was populated
with two sets of signals having same number of protons. By comparing with the spectrum
of L1 in same solvent, it has been observed that the peaks at 1.20-1.23 (t, 6H), 2.00 (s,
6H), 2.58-2.62 (q, 4H) and 3.66 (s, 2H) ppm corresponds to L1. The other set {0.99 (t,
6H), 2.34 (s, 10H) and 3.92 (s, 2H) ppm} shows a slight shift of the peak positions which
is consistent with the metal bound ligand in Co(I)-dinitrosyl complex, 2.2. Thus, 2.2 was
+ formulated as [(L1)Co(NO)2] . The addition of one more equivalent of NO again resulted
in one set of peaks with paramagnetic broadening (Figure 2.5). This spectrum matches well
with the spectrum of isolated 2.3 (Appendix I).
22
TH-1730_126122002
Chapter 2
The formulation of 2.2 was further supported by ESI-mass spectrometric analyses. The
addition of 3 equivalents of NO to the solution of 2.1 resulted in a spectrum populated by a
+ peak at m/z 351.02 (Figure 2.4b). This is assignable to the [(L1)Co(NO)2] unit. The peak
was found to be isotope sensitive and shifted to m/z, 353.34 upon labelling with 15NO
+ which is again consistent with the mass of [(L1)Co(NO)2] moiety .The simulated isotopic
distribution pattern was found to be in good agreement with the observed one (Figure
2.4b). In the presence of further equivalent of NO, the peak at m/z, 351.02 disappeared and
+ a new peak at m/z, 523.23 corresponding to [(L1)2Co] was observed (Appendix I).
Figure 2.5. 1H-NMR spectra of (a) ligand L1 (black), (b) complex 2.1 (violet), (c) after addition of
3 equivalents of NO, 2.2+L1 and (d) after addition of excess NO, complex 2.3 in CD3OD. The * marked signals are for complex 2.2.
Further, the formation of 2.3 in the reaction was confirmed by its isolation and structural
characterization (Experimental section). The ORTEP diagram of 2.3 is shown in figure
2.1b. The crystal structure revealed that Co(II) is co-ordinated with four N-atom from two
- units of ligand and two O-atoms from a nitrite anion (NO2 ) in an distorted octahedral
23
TH-1730_126122002
Chapter 2
geometry. The Co-Onitrito bond distance is 2.23 (3) Å, and Co-O-N bond angle is 98.0 (3)º
which is consistent with the previously reported CoII-nitrito complexes.
Thus, a total of 4 equivalents of NO is required to result in the formation 2.3 from 2.1 via
the intermediate formation of 2.2 (Scheme 2.1). The reaction of 2.1 with NO can be
envisaged as the reductive elimination in the first step leading to the formation of 2.2,
+ [(L1)Co(NO)2] (Scheme 2.1). In this case one equivalent of NO is required for the
reduction of Co(II) to Co(I) and other 2 equivalents of NO form the dinitrosyl complex in
subsequent step. The NO induced reduction of Co(II) was confirmed by the presence of
MeONO in the reaction mixture in GC-mass analyses (Appendix I). In the next step, the
dinitrosyl complex reacts with equivalent amount of NO to result in the nitrito complex,
2.3 (Scheme 2.1).
Scheme 2.1. Reaction of complex 2.1 with NO.
The reaction of free NO with the metal nitrosyls was reported previously. In this context,
the Tolman’s group observed that a Cu(I) complex of pyrazolyl borate derivative ligand,
reacts with NO to form a Cu-dinitrosyl (or Cu-hyponitrite) complex and a subsequent
24
TH-1730_126122002
Chapter 2
equivalent of NO resulted in the corresponding Cu(II)-nitrite.25 A simultaneous release of
N2O was observed in this reaction. In addition, Fe(TC-5,5) and Mn(TC-5,5) were reported
to react with stoichiometric 4 equivalents of NO leading to the formation of
26 corresponding O-bound metal nitrite and N2O. Thus, in the present case, the release of
N2O is expected and its presence was confirmed by the head space gas analysis using GC-
mass spectrometry (Appendix I).
2.3 Experimental section
2.3.1 Materials and methods
All reagents and solvents were purchased from commercial sources and were of reagent
grade. HPLC grade methanol was used and dried by heating with iodine-activated
magnesium with magnesium loading of ca. 5 g/L. The dried methanol was stored over
20% (m/v) molecular sieves (3 Å) for 3 days before use. Deoxygenation of the solvent and
solutions were effected by repeated vacuum/purge cycles or bubbling with nitrogen or
argon for 30 minutes. NO gas was purified by passing through KOH and P2O5 column. The
dilution of NO was effected with argon gas using Environics Series 4040 computerized gas
dilution system. UV-visible spectra were recorded on an Agilent HP 8454
spectrophotometer. FT-IR spectra were taken on a Perkin-Elmer spectrophotometer with
either sample prepared as KBr pellets or in solution in a KBr cell. 1H-NMR spectra were
obtained with a 400 MHz Varian FT spectrometer. Chemical shifts (ppm) were referenced
either with an internal standard (Me4Si) for organic compounds or to the residual solvent
peaks. The X-band Electron Paramagnetic Resonance (EPR) spectra were recorded on a
JES-FA200 ESR spectrometer, at room temperature or at 77 K. Spectra were recorded
with all the samples prepared in methanol solution at 77 K. The experimental conditions
[Frequency, 9.136 GHz; Power, 0.995 mW; Field center, 336.00 mT, Width, 250.00 mT;
25
TH-1730_126122002
Chapter 2
Sweep time, 30.0 s; Modulation frequency, 100.00 kHz, Width, 1.000 mT; Amplitude, 1.0
and Time constant, 0.03 s] kept same for all the samples. Mass spectra of the compounds
were recorded in a Waters Q-Tof Premier and Aquity instrument. Solution electrical
conductivity was checked using a Systronic 305 conductivity bridge. The magnetic
moment of complexes were measured on a Cambridge Magnetic Balance. Elemental
analyses were obtained from a Perkin Elmer Series II Analyzer.
Single crystals were grown by slow evaporation technique. The intensity data were
collected using a Bruker SMART APEX-II CCD difractometer, equipped with a fine focus
1.75 kW sealed tube Mo Kα radiation (λ = 0.71073 Å) at 273(3) K, with increasing ω
(width of 0.3° per frame) at a scan speed of 3 s/frame. The SMART software was used for
data acquisition.27 Data integration and reduction was undertaken with SAINT and XPREP
software. Structures were solved by direct methods using SHELXS-97 and refined with
full-matrix least squares on F2 using SHELXL-97.28 Structural illustrations have been
drawn with ORTEP-3 for Windows.29
2.3.2 Syntheses
2.3.2.1 Synthesis of ligand L1, [bis(2-ethyl-4-methylimidazol-5-yl)methane]
2-ethyl-4-methyl imidazole (2.2 g, 20 mmol) was taken in a round bottom flask with 50
mL of methanol; to this 0.45 g (15 mmol) of formaldehyde and aqueous solution of 5.1 g
(91 mmol) potassium hydroxide was added drop wise. The reaction mixture was stirred for
24 h at room temperature. A white solid was obtained. It was filtered off and washed with
water. After washing the solid was dried in air and then kept in desiccator for overnight.
Yield: 1.97 g (85%). Elemental analyses for C13H20N4, Calcd(%): C, 67.21; H, 8.68; N,
24.12. Found (%): C, 67.19; H, 8.68; N, 24.02. FT-IR (in KBr): 2964, 2842, 1614, 1529,
-1 1 1455, 1071 cm . H-NMR (400 MHz, CDCl3): δppm, 1.20-1.23 (t, 6H), 2.00 (s, 6H), 2.58-
26
TH-1730_126122002
Chapter 2
13 2.62 (q, 4H) and 3.66 (s, 2H). C-NMR (100 MHz, CDCl3): δppm, 149.2, 129.1, 127.2,
23.3, 22.6, 13.7, 10.6.
2.3.2.2 Syntheses of complexes
2+ (a) 2.1, [(L1)2Co]
A solution of CoCl2 .6H2O (1.19 g , 5 mmol) in methanol (20 mL) was added to a solution
of the ligand (2.32 g, 10 mmol) in methanol (15 mL) over a period of 10 min. The resulting
violet solution was stirred for 3 h and then kept at room temperature for slow evaporation.
After a few days, violet color crystals of 2.1 were obtained. Yield: 2.43 g (82%). Elemental
analyses for C26H40Cl2CoN8, Calcd. (%): C, 52.53; H, 6.78; N, 18.85, found (%): C, 52.47;
H, 6.80; N, 18.93. UV-visible (MeOH): 319 nm (ε/M-1cm-1, 772), 515 nm (ε/M-1cm-1, 323),
559 nm (ε/M-1cm-1, 494), and 581 nm (ε/M-1cm-1, 490). FT-IR (in KBr): 2975, 1603, 1465,
-1 1295, 1079, 825, 674 cm . X-band EPR: gav 2.29. Molar conductance (MeOH): 185
2 -1 Scm mol . Observed Magnetic moment: 4.32 µB. ESI-Mass (m/z): Calcd: 261.63, found:
261.60 (M/2).
+ (b) 2.3, [(L1)2Co(NO2)]
Complex 2.1 was dissolved in 30 mL of dry and degassed methanol in a 50 mL schlenk
flask fitted with a rubber septum connected to the schlenk line. The solution was degassed
by argon gas through vacuum purge continuously for 10 min. 4 equivalents of NO was
added to the solution and stirred at room temperature for 1h, after a few days a brown
colour crystals of 2.3 were obtained. Elemental analyses for C26H40ClCoN9O2, Calcd (%):
C, 51.61; H, 6.66; N, 20.84, found (%): C, 51.57; H, 6.71; N, 20.87. UV-visible (MeOH):
523 nm (ε/M-1cm-1, 184), 558 nm (ε/M-1cm-1, 312), 579 nm (ε/M-1cm-1, 360). FT-IR (in
27
TH-1730_126122002
Chapter 2
-1 KBr): 1634, 1535, 1452, 1428, 1323, 1261, 1208, 1073, 1050 cm . X-band EPR: gav 1.89.
2 -1 Molar Conductance: 135 Scm mol . Observed magnetic moment: 1.78 µB.
2.4 Conclusion
In conclusion, the present study demonstrates an example of the reaction of a Co(II)
complex, 2.1 with 3 equivalents of NO to result in a Co-dinitrosyl complex, 2.2 via the
reductive nitrosylation pathway. The addition of further equivalent of NO to the reaction
mixture afforded the nitrito complex, 2.3 simultaneous release of N2O. Spectroscopic
analyses and isotope labelling experiments revealed the formulation of 2.2 as
+ [(L1)Co(NO)2] , whereas structural characterization confirmed the formation of 2.3.
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1984, 106, 7462.
(11) (a) Richter-Addo, G. B.; Hodge, S. J.; Yi, G.-B.; Khan, M. A.; Ma, T.; Van
Caemelbecke, E.; Guo, N.; Kadish, K. M. Inorg. Chem. 1996, 35, 6530. (b) Di
Vaira, M.; Ghilardi, C. A.; Sacconi, L. Inorg. Chem. 1976, 15, 1555. (c)
Thyagarajan, S.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. Inorg. Chim.
Acta 2003, 345, 333.
(12) Rhine, M. A.; Rodrigues, A. V.; Urbauer, R. L. B.; Urbauer, J. L.; Stemmler, T. L.;
Harrop, T. C. J. Am. Chem. Soc. 2014, 136, 12560.
(13) Kalita, A.; Kumar, P.; Deka, R. C.; Mondal, B. Inorg. Chem. 2011, 50, 11868.
(14) (a) Kozhukh, J.; Lippard, S. J. J. Am. Chem. Soc. 2012, 134, 11120. (b) Franz, K.
J.; Doerrer, L. H.; Springler, B.; Lippard, S. J. Inorg. Chem. 2001, 40, 3774. (c)
Scott, M. J.; Lippard, S. J. J. Am. Chem. Soc. 1997, 119, 341. (d) Villacorta, G. M.;
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Lippard, S. J. Pure Appl. Chem. 1986, 58, 1474. (e) Zask, A.; Gonnella, N.;
Nakanishi, K.; Turner, C. J.; Imajo, S.; Nozoe, T. Inorg. Chem. 1986, 25, 3400. (f)
Jaynes, B. S.; Doerrer, L. H.; Liu, S.; Lippard, S. J. Inorg. Chem. 1995, 34, 5735.
(15) (a) Kalita, A.; Kumar, P.; Deka, R. C.; Mondal, B. Chem. Commun. 2012, 48,
1251. (b) Ghosh, S.; Deka, H.; Dangat, Y. B.; Saha, S.; Gogoi, K.; Vanka, K.;
Mondal, B. Dalton Trans. 2016, 45, 10200.
(16) Song, Y. S.; Kofod, P.; Madsen, A. S.; Larsen, E. Inorg. Chem. 2009, 48, 7159.
(17) (a) Jesson. J. P. J. Chem. Phys. 1968, 48, 161. (b) Drake, A. F.; Hirst, S. J.; Kuroda,
R.; Mason, S. F. Inorg. Chem. 1982, 21, 533. (c) Dzwigaj, S.; Che, M. J. Phys.
Chem. B 2006, 110, 12490.
(18) Romerosa, A.; Bello, C. S.; Ruiz, M. S.; Caneschi, A.; McKee, V.; Peruzzini, M.;
Sorace, L.; Zanobini, F. Dalton Trans. 2003, 3233.
(19) Fujisawa, K.; Soma, Shoko.; Kurihara, H.; Dong, H. T.; Bilodeaub, M.; Lehnert, N.
Dalton Trans. 2017, 46, 13273.
(20) Deka, H.; Ghosh, S.; Saha, S.; Gogoi, K.; Mondal, B. Dalton Trans. 2016, 45,
10979.
(21) (a) Sarma, M.; Kalita, A.; Kumar, P.; Singh, A.; Mondal, B. J. Am. Chem. Soc.
2010, 132, 7846. (b) Sarma, M.; Mondal, B. Inorg. Chem. 2011, 50, 3206. (c)
Sarma, M.; Mondal, B. Dalton Trans. 2012, 41, 2927. (d) Haymore, B. L.;
Huffman, J. C.; Butler, N. E. Inorg. Chem. 1983, 22, 168.
(22) (a) Franz, K. J.; Singh, N.; Spingler, B.; Lippard, S. J. Inorg. Chem. 2000, 39,
4081. (b) Tomson, N. C.; Crimmin, M. R.; Petrenko, T.; Rosebrugh, L. E.;
Sproules, S.; Boyd, W. C.; Bergman, R. G.; DeBeer, S.; Toste, F. D.; Wieghardt, K.
J. Am. Chem. Soc. 2011, 133, 18785. (c) Hilderbrand, S. A.; Lippard, S. J. Inorg.
Chem. 2004, 43, 5294. (d) Lim, M. H.; Lippard, S. J. Acc. Chem. Res. 2007, 40, 41.
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(23) (a) Goodwin, J.; Kurtikyan, T.; Standard, J.; Walsh, R.; Zheng, B.; Parmley, D.;
Howard, J.; Green, S.; Mardyukov, A.; Przybyla, D. E. Inorg. Chem. 2005, 44,
2215. (b) Gogoi, K.; Deka, H.; Kumar, V.; Mondal, B. Inorg. Chem. 2015, 54,
4799. (c) Lehnert, N.; Cornelissen; Neese, F.; Ono, T.; Noguchi, Y.; Okamoto, K.-
I.; Fujisawa, K. Inorg. Chem. 2007, 46, 3916.
(24) Sengupta, A.; Rajput, A.; Barman, S. K.; Mukherjee, R. Dalton Trans. 2017, 46,
11291.
(25) Schneider, J. L.; Carrier, S. M.; Ruggiero, C. E.; Young, J. V. E.; Tolman, W. B. J.
Am. Chem. Soc. 1998, 120, 11408.
(26) (a) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504. (b) Franz, K. J.;
Lippard, S. J. J. Am. Chem. Soc. 1998, 120, 9034.
(27) SMART, SAINT and XPREP, Siemens Analytical X-ray Instruments Inc.,
Madison, Wisconsin, U. S. A. 1995.
(28) (a) Sheldrick, G. M.; SADABS: software for Empirical Absorption Correction,
University of Gottingen, Institute for Anorganische Chemieder Universitat,
Tammanstrasse 4, D-3400 Gottingen, Germany 1999. (b) Sheldrick, G. M.
SHELXS-97, University of Gottingen, Germany 1997.
(29) Farrugia, L. J. ORTEP-3 for windwos-a version of ORTEP-III with a graphical user
interface (GUI). J. Appl. Crystallogr. 1997, 30, 565.
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TH-1730_126122002 Chapter 3
Reactivity of a Cobalt Nitrosyl Complex: Formation of a Cobalt Peroxynitrite Intermediate and Transfer of the Nitrosyl to a Cobalt Porphyrin Complex
Abstract
A cobalt salen complex, [(L2)Co], 3.1 {L2H2 = 6,6'-((1E,1'E)-(ethane-1,2-
diyl)bis(azanylylidene))bis(methanylylidene))bis(2,4-di-tert-butylphenol} in THF solution
was subjected to react with nitric oxide (NO) gas and resulted in the formation of the
corresponding nitrosyl complex, [(L2)Co(NO)], 3.2 having {CoNO}8 description. It was
characterized by spectroscopic studies and single crystal X-ray structure determination. It
did not react with dioxygen. However, in THF solution, it reacted with KO2 to result in the
- Co-nitrate complex, [(L2)Co(NO3)] , 3.3. It induced ring nitration to the externally added
phenol in an appreciable yield. The reaction presumably proceeds through the formation of
corresponding Co-peroxynitrite intermediate. In addition, complex 3.2 in THF solution,
was found to transfer the NO group to a cobalt porphyrin complex, [(L3)Co], 3.4 {L3 =
5,10,15,20-tetrakis(4'-bromophenyl)porphyrinate dianion}to result in the corresponding
nitrosyl complex, [(L3)Co(NO)], 3.5.
TH-1730_126122002
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3.1 Introduction
Nitric oxide (NO) is an important small molecule that is involved in various physiological
process; e.g. neurotransmission, vasodilation, immune response etc.1-4 Under normal
conditions, NO is produced by nitric oxide synthase (NOS) at low concentration. However,
excess production of NO can have detrimental effects via the formation of secondary
reactive nitrogen species (RNS) such as nitrogen dioxide (NO2) and peroxynitrite
(ONOO-).5 Both of these RNS are known to play important roles in bio-molecule
oxidation leading to the oxidative and nitrosative stress.6 The formation of these secondary
intermediates from NO requires the presence of oxidants like hydrogen peroxide (H2O2),
- 7,8 superoxide (O2 ) radicals and transition metal centers. Peroxynitrite is believed to form
- in vivo in the diffusion controlled reaction of NO and superoxide anion (O2 ) or hydrogen
- 9 peroxide (H2O2) and nitrite (NO2 ) in presence of peroxidase enzymes. NO2 forms either
- from the homolytic O-O cleavage of ONOO or by the reaction of NO with O2. Aqueous
- - ONOO isomerizes to nitrate (NO3 ) very easily. Peroxynitrite is reported to from in the
III - reaction of oxy-heme (Fe -O2 ) proteins with NO and superoxo-iron or -cobalt
porphyrinates with NO. Recently ONOO- intermediates are shown to from in the reaction
of NO with Cr-superoxo or peroxo species; a Cr(IV)-peroxo complex, [(12-
+ TMC)Cr(O2)(Cl)] reacted with NO to form a Cr(III)-nitrato complex, [(12-
+ + TMC)Cr(NO3)(Cl)] , whereas a Cr(III)-superoxo complex, [(14-TMC)Cr(O2)(Cl)] and
+ NO gave a Cr(IV)-oxo complex, [(14-TMC)Cr(O)(Cl)] and NO2 presumably via the
formation of a Cr(III)-peroxynitrite intermediate, [(14-TMC)Cr(OONO)(Cl)]+).10-12
Recently Nam and his co worker has reported two mononuclear Co(III)-nitrosyl
complexes, [(12-TMC)Co(NO)]2+ and [(13-TMC)Co(NO)]2+, and examined their reactions
- II with O2 to give high yields of Co -nitrito complexes with the release of O2 via the
formation of presumed Co(II)-(ONOO-) intermediates.13
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On the other hand, another important aspect of metal-nitrosyl is the transfer of NO centre
from one metal to another. This depends on the NO-to-metal binding constant, ligand
geometry, and metal oxidation state. Caulton and co-workers have reported nitrosyl
transfer from [(DMGH)2Co(NO)] (DMGH = monoanion of dimethylglyoxime) to the
various metal complexes of Fe, Co, Ni, and Ru.14 Lippard and co-workers also suggested a
dissociative pathway of NO-transfer from manganese to iron bearing a tropocoronand
ligand framework.15 Very recently Nam group have reported the NO-transfer from [(14-
TMC)Co(NO)]2+ to [(12-TMC)Co]2+.16
The present work describes the reactivity of a Co-nitrosyl complex of salen type ligand
with i) superoxide ion to form Co-nitrate complex via a putative peroxynitrite intermediate
and ii) NO transfer to a Co-porphyrin complex.
3.2 Results and discussion
The ligand L2H2 {L2H2 = 6,6'-((1E,1'E)-(ethane-1,2-diyl)bis(azanylylidene))-bis
(methanylylidene))bis(2,4-di-tert-butylphenol)} was prepared by following an earlier
reported procedure of salen ligand preparation. The Co(II) complex, 3.1 was prepared by
stirring a mixture of cobalt(II) acetate tetrahydrate with one equivalent of L2H2 in
methanol and diethyl ether mixture. Isolated complex 3.1 was characterized by various
spectroscopic analyses as well as the single crystal X-ray structure determination. The
crystallographic data and other metric parameters listed in appendix II. The perspective
ORTEP view of the 3.1 is shown in figure 3.1. Structural characterization revealed that
Co(II) ion is coordinated by two Nimine and two Ophenol atoms from the ligand in an overall
distorted square planar geometry. The average Co-Nimine and Co-Ophenolato distances are
1.848 and 1.845 Å, respectively, in 3.1. These are within the range found in earlier
35
TH-1730_126122002
Chapter 3
reported examples.17 The phenol moieties of the ligands are coordinated to the metal ion as
phenolate and thereby neutralizing the overall charge of the metal ions.
(a) (b)
Figure 3.1. ORTEP diagram of complexes (a) 3.1 and (b) 3.2 (30% thermal ellipsoid plot. H- atoms are not shown for clarity).
NO reactivity of complex 3.1
In dry and degassed THF solution of 3.1, NO gas was purged for ~5 min and the red color
of the solution became dark brown which corresponds to complex 3.2. It was isolated by
precipitation and characterized via various spectroscopic analyses as 3.2. In UV-visible
spectroscopy, the absorption bands at 406 nm (/M-1cm-1, 1640) and 711 nm (/M-1cm-1,
45) of 3.1 were diminished upon addition of NO (Figure 3.2a).
(a) (b)
Figure 3.2. (a) UV-visible spectra of complex 3.1 (black) and after addition of NO, 3.2 (red). (b) FT-IR spectrum of complex 3.2 in KBr.
36
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Chapter 3
In FT-IR spectroscopy, complex 3.2 shows a strong stretching frequency at 1650 cm-1
which is attributed to the N-O stretching frequency of a coordinated nitrosyl group (Figure
3.2b). In case of [(12-TMC)Co(NO)]2+ and [(13-TMC)Co(NO)]2+, these nitrosyl stretching
frequencies appeared at 1712 and 1716 cm-1 in solution FT-IR spectroscopy.13 In X-band
EPR spectroscopy, 3.2 was found to be silent in THF solution at room temperature as well
as at 77 K (Appendix II). ESI mass spectrum of 3.2 displayed peaks assignable to the
[(L2)Co]+ fragments (Appendix II). This is attributed to the facile loss of the NO group as
axial ligand under reaction condition. This was observed earlier with cobalt nitrosyls of
porphyrin ligands.18 Further, 3.2 was characterized by single crystal X-ray structure
determination (Figure 3.1b). The crystal structure reveals that Co(II) centre is coordinated
by two nitrogen atoms and two phenolato oxygen atoms of the ligand and one N-atom
from nitrosyl in an overall square pyramidal geometry. The NO ligand is axially
coordinated to the cobalt center in a bent fashion. The crystallographic data and other
metric parameters listed in Appendix II. The N-O bond length is 1.155 Å and the large
bending of the metal-nitrosyl moiety with Co-N-O bond angle is 126.47° is also consistent
with coordinated {CoNO}8.19
Reactivity of complex 3.2 with potassium superoxide (KO2)
It was reported earlier that cobalt-nitrosyls react with molecular O2 leading to formation of
20 corresponding nitrite complexes. However, 3.2 did not react with O2. Similar observation
was reported with Co(III)-nitrosyls, [(12-TMC)Co(NO)]2+ and [(13-TMC)Co(NO)]2+.13 In
those cases it was speculated that NO did not undergo dissociation in the complexes which
13 is required for the O2 reactivity. However, addition of KO2 in presence of 18-crown-6 in
dry THF solution 3.2 afforded a change in color. In the UV-visible spectroscopy, a new
absorption band at 665 nm (/M-1cm-1, 95) was observed, this is attributed to the formation
of corresponding Co(III)-nitrato complex, 3.3 (Figure 3.3a). In FT- IR the 1650 cm-1
37
TH-1730_126122002
Chapter 3
nitrosyl stretching frequency disappeared and a new stretching frequency at 1385 cm-1
- appeared which is assignable to the NO3 stretching frequency (Figure 3.3b). Even after
several attempts we could not get any X-ray quality crystal of 3.3. Other spectral and
elemental analyses confirm the formation of 3.3 unambiguously. However, the formation
of 3.3 suggests the formation of a putative ONOO- intermediate in the course of the
- - reaction (Scheme 3.1) as NO3 is the common isomerisation product of ONOO .
(a) (b)
Figure 3.3. (a) UV-visible spectra of complex 3.2 (black) and after addition of KO2, 3.3 (red) in THF. (b) FT-IR spectrum of complex 3.3 in KBr.
Since direct spectroscopic evidence of the ONOO- intermediate could not be obtained
owing to its unstable nature, the reported methods for identification of ONOO- have been
12b followed. When the reaction of 3.2 with KO2 is carried out at -40 °C followed by the
addition of 2,4-di-tert-butylphenol (DTBP), complex 3.3 is formed and unreacted DTBP
is recovered. However, effective nitration (ca. 50%) is observed when DTBP is added
before the addition of KO2 (Scheme 3.1 and Experimental section). In this connection,
nitrosyls of copper(II) complexes with methyl 2-(2-hydroxybenzylamino)-3-(1H-imidazol-
5-yl)propanoate21 and bis(2-ethyl-4-methyl-imidazol-5yl)methane22 ligands were reported
to react with H2O2 to result in the corresponding nitrate products. They were also found to
38
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Chapter 3
induce the phenol ring nitration reaction. In both the cases, the involvement of the ONOO-
intermediate was presumed.
- Scheme 3.1. Formation of ONOO intermediate in the reaction of complex 3.2 with KO2 in THF at -40 °C.
NO transfer reaction
The NO transfer from 3.2 to 3.4, [(L3)Co] {L3 = 5,10,15,20-tetrakis(4'-
bromophenyl)porphyrinate dianion} was studied (Scheme 3.2). When equivalent amount
of 3.2 was subjected to mix with 3.4 in dry and degassed THF in Ar atmosphere, a dark red
solution was formed. In UV-visible spectroscopy the 412 nm (ε/M-1cm-1, 2.01 × 105) and
527 nm (ε/M-1cm-1, 2.8 × 104) bands of 3.4 were found to shift 416 nm (ε/M-1cm-1, 1.85 ×
105) and 540 nm (ε/M-1cm-1, 2.6 × 104), respectively (Figure 3.4a). In solution FT-IR
spectroscopy the appearance of a new stretching at 1720 cm-1 suggest the formation of
complex 3.5 [(L3)Co(NO)] (Figure 3.4b).
The mixture was separated by column chromatography and found as complexes 3.1 and
3.5. Both the complexes were isolated as solid and characterized via spectral analyses and
structure determination.
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TH-1730_126122002
Chapter 3
Scheme 3.2. Transfer of nitrosyl group from complex 3.2 to 3.4.
(a) (b)
Figure 3.4. (a) UV-visible spectral monitoring of the reaction of complex 3.2 with 3.4 (black) to result 3.5 (red) in THF. (b) Solution FT-IR monitoring of the reaction of complex 3.2 (black) with 3.4 to result 3.5 (blue) in THF.
The single crystal X-ray structure of 3.5 reveals that the NO ligand is axially coordinated
to the cobalt center in a bent fashion in an overall distorted square pyramidal geometry
(Figure 3.5b). The crystallographic data and other metric parameters listed in appendix II.
The N-O bond length is 1.195 Å and Co-N-O bond angle is 119.42° which is consistent
40
TH-1730_126122002
Chapter 3
with coordinated {CoNO}8.19 In a similar manner the NO transfer was reported from [(14-
TMC)Co(NO)]2+ to [(12-TMC)Co]2+.16
(a) (b)
Figure 3.5. ORTEP diagram of complexes (a) 3.4 and (b) 3.5. (30% thermal ellipsoid plot. H- atoms are not shown for clarity).
3.3 Experimental section
3.3.1 Materials and methods
All reagents and solvents of reagent grade were purchased from commercial sources and
used as received except specified. Deoxygenation of the solvent and solutions was effected
by repeated vacuum/purge cycles or bubbling with nitrogen or argon for 30 min. NO gas
was used from a cylinder after purification using standard procedure. The dilution of NO
was effected with argon gas using Environics Series 4040 computerized gas dilution
system. UV-visible spectra were recorded on Agilent HP 8454 dioad arrey UV-visible
spectrophotometer. FT-IR spectra of the solid samples were taken on a Perkin Elmer
spectrophotometer with samples prepared as KBr pellets. Solution electrical conductivity
was measured using a Systronic 305 conductivity bridge. 1H-NMR and 13C-NMR spectra
were recorded respectively in 400 MHz or 600 MHz and 100 MHz Varian FT
spectrometer. Chemical shifts (ppm) were referenced either with an internal standard
(Me4Si) or to the residual solvent peaks. The X-band Electron Paramagnetic Resonance
41
TH-1730_126122002
Chapter 3
(EPR) spectra were recorded on a JES-FA200 ESR spectrometer, at room temperature or at
77 K with microwave power, 0.998 mW; microwave frequency, 9.14 GHz and modulation
amplitude, 2. Elemental analyses were obtained from a Perkin Elmer Series II Analyzer.
The magnetic moment of complexes was measured on a Cambridge Magnetic Balance.
Single crystals were grown by slow diffusion followed by slow evaporation technique. The
intensity data were collected using a Bruker SMART APEX-II CCD diffractometer,
equipped with a fine focus 1.75 kW sealed tube MoK radiation ( = 0.71073 Å) at 273(3)
K, with increasing (width of 0.3° per frame) at a scan speed of 3 s/frame. The SMART
software was used for data acquisition.23 Data integration and reduction were undertaken
with SAINT and XPREP software.24 Structures were solved by direct methods using
SHELXS-97 and refined with full-matrix least squares on F2 using SHELXL-97.25
Structural illustrations have been drawn with ORTEP-3 for Windows.26
3.3.2 Syntheses
3.3.2.1 Syntheses of ligands
(a) L2H2, [6,6'-((1E,1'E)-(ethane-1,2-diyl)bis(azanylylidene))bis (methanylylidene))bis-
(2,4-di-tert-butylphenol)]
To the solution of ethylenediamine (0.6 g, 10 mmol) in 10 mL ethanol, 20 mL ethanol
solution of 3,5-di-tert-butylsalicylaldehyde (4.68 g, 20 mmol) was added drop wise at
room temperature. The reaction mixture was allowed to stir for 4 h till yellow colored
precipitate formed. It was filtered off and washed with cold ethanol. After washing the
solid was dried in air and kept in desiccator for overnight. Yield: 4.1g (83%). Elemental
analyses for C32H48N2O2, Calcd. (%): C, 78.00; H, 9.82; N, 5.69. Found (%): C, 77.95; H,
9.79; N, 5.78. FT-IR (in KBr): 2961, 2869, 1629, 1481, 1466, 1439, 1361, 1270, 1253,
-1 1 1041, 879, 839, 830, 773, 729, 710, 645 cm . H-NMR (400 MHz, CDCl3): δppm, 1.23 (s,
42
TH-1730_126122002
Chapter 3
9H), 1.41 (s, 9H), 3.92 (s, 1H), 7.06 (s, 1H), 7.36 (s, 1H), 8.38 (s, 1H), 13.62 (s, 1H). 13C-
NMR (100 MHz, CDCl3): δppm, 29.6, 31.7, 34.3, 59.8, 118.0, 126.3, 127.2, 136.8, 140.3,
158.2, 167.8.
(b) L3H2, [5,10,15,20-tetrakis(4'-bromophenyl)porphyrin]
The ligand L3H2 was prepared by following earlier reported general procedure of
porphyrin synthesis with slight modification.28 p-Bromobenzaldehyde (3.70 g, 20 mmol)
and freshly distilled pyrrole (1.34 g, 20 mmol) were added to 37 mL of propionic acid.
After refluxing for 2 h the solution was cooled to room temperature. It was then
neutralized by aqueous Na2CO3. A precipitate appeared and was filtered off. The crude
mass was subjected to purification using column chromatography using CH2Cl2 and
hexane as solvent to afford the desired ligand as purple solid. Yield: 0.697 g (20%).
Elemental analyses for C44H26N4Br4. Calcd. (%): C, 56.81; H, 2.82; N, 6.02, found (%): C,
56.65; H, 2.88; N, 6.07. FT-IR (in KBr): 3439, 2922, 1473, 1394, 1071, 1010, 962, 803,
-1 1 730 cm . H-NMR (600 MHz, CDCl3): δppm, 7.90-7.91 (d, 8H), 8.06-8.07 (d, 8H), 8.84 (s,
8H). Mass (m/z): Calcd: 929.885, found: 930.867 (M+1).
3.3.2.2 Syntheses of complexes
(a) 3.1, [(L2)Co]
Complex 3.1 was prepared by following the reported procedure with some
modifications.27 Cobalt(II) acetate tetrahydrate (1.25 g, 5 mmol) was taken in a 50 mL
round bottom flask, dissolved in 10 mL of MeOH. To this, a solution of ligand L2H2 (2.46
g, 5 mmol) in hot methanol (25 mL) was added and stirred for 3 h. A red color precipitate
was formed, it was filtered off and washed several time by cold methanol. After washing
the solid was dried in air and then kept in desiccator for overnight. Finally it was
recrystallized from THF solution to obtain red crystals. Yield: 1.77 g (72%). Elemental
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TH-1730_126122002
Chapter 3
analyses for C32H46N2O2Co, Calcd (%): C, 69.92; H, 8.44; N, 5.10. Found (%): C, 69.80;
H, 8.41; N, 5.16. FT-IR (in KBr): 3433, 2952, 1598, 1528, 1462, 1439, 1254, 1175, 872,
786 cm-1. UV-visible (THF): 406 nm (/M-1cm-1, 1640) and 711 nm (/M-1cm-1, 45). Molar
2 -1 conductance (THF): 38 Scm mol observed magnetic moment: 1.55 µB.
(b) 3.2, [(L2)Co(NO)]
In 20 mL of dry and degassed THF solution of 3.1 (275 mg, 0.5 mmol), freshly purified
NO was bubbled till greenish brown color was appeared. The reaction mixture was kept at
room temperature for crystallization. Yield: 0.250 g (88%). Elemental analyses for
C32H46N3O3Co, Calcd (%): C, 66.31; H, 8.00; N, 7.25. Found (%): C, 66.24; H, 8.02; N,
7.34. FT-IR (in KBr): 2955, 1649, 1620, 1529, 1430, 1313, 1172, 1090, 744, 530 cm-1.
- (c) 3.3, [(L2)Co(NO3)]
Complex 3.3 was prepared from freshly prepared complex 3.2, (290 mg, 0.5 mmol) and 2
equivalent of KO2 in presence of 18-crown-6 (5 equivalent) in dry THF at -80°C. The
volume of the reaction mixture was reduced to 5 mL using vacuum and layered with
hexane. The reaction mixture was kept in freezer for 24 h. The dark brown precipitate was
filtered off and washed with hexane for several times. The solid mass was kept at
desiccator for overnight. Yield: 0.195 g (64%). Elemental analyses for C32H46N3O5Co,
Calcd (%): C, 62.84; H, 7.58; N, 6.87. Found (%): C, 62.77; H, 7.69; N, 6.88. FT-IR (in
KBr): 1630, 1613, 1434, 1385, 1359, 1321, 1255, 1170, 1046, 834, 784, 540 cm-1. UV-
visible (THF): 665 nm (/M-1cm-1, 95). Molar conductance (THF): 35 Scm2mol-1.
(d) 3.4, [(L3)Co]
To a mixture of ligand, L3H2 (0.232 g, 0.25 mmol) in CHCl3 (12 mL) and AcOH (12 mL),
Co(OAc)2.4H2O (0.622 g, 2.50 mmol) was added. After refluxing at 120 °C for 2 h, the
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Chapter 3
precipitate was filtered and washed with MeOH (15 mL × 4). The crude mass was
subjected to column chromatography to result in complex 3.4 as violet solid. Yield: 0.160
g (65%). Elemental analyses for C44H24N4Br4Co, Calcd. (%): C, 53.53; H, 2.45; N, 5.68,
found (%): C, 53.48; H, 2.49; N, 5.71. UV-visible (THF): 412 nm (ε/M-1cm-1, 2.01 × 105),
527 nm (ε/M-1cm-1, 2.8 × 104) . FT-IR (in KBr): 1541, 1483, 1347, 1071, 1001, 800, 719,
486 cm-1. Mass (m/z): Calcd. 986.80, found: 986.50 (molecular ion peak).
(e) 3.5, [(L3)Co(NO)]
In 20 mL of dry and degassed THF solution of 3.2 (0.290 g, 0.5 mmol), a solution of 3.4
(490 mg, 0.5 mmol) in 50 mL of dry THF was added. After stirring at room temperature
for 2 h the mixture was dried under reduced pressure. The crude mass was subjected to
column chromatography using CHCl3 and hexane as eluent to result in complex 3.5.
Elemental analyses for C44H24N5OBr4Co, Calcd. (%): C, 51.95; H, 2.38; N, 6.88, found
(%): C, 51.78; H, 2.47; N, 6.76. UV-visible (THF): 416 nm (ε/M-1cm-1, 1.85 × 105), 540
nm (ε/M-1cm-1, 2.6 × 104). FT-IR (in KBr): 1720, 1480, 1348, 1345, 1070, 1008, 805, 762,
-1 1 713, 480 cm . H-NMR (600 MHz, CDCl3): δppm, 7.92-7.89 (d, 8H), 8.05-8.02 (d, 8H),
8.90 (s, 4H), 9.02 (s, 4H).
3.3.2.3 Reaction of complex 3.2 with KO2 in presence of 2,4-di-tert-butylphenol
In 20 mL THF solution of freshly prepared complex 3.2 (0.290 g, 0.5 mmol), a solution of
2,4-di-tert-butyl phenol (DTBP) (0.620 g, 3 mmol in 5 mL dry THF), was added and the
mixture was cooled to -40 ºC. To that KO2/18-crown-6 (~1 equivalent) was added, and the
mixture was stirred for 20 min. After that, it was warmed to room temperature and dried
under reduce pressure. The solid mass was then subjected to column chromatography using
silica gel column and hexane as solvent to obtain 2, 4-di-tert-butyl-6-nitrophenol. Yield:
0.063 g (50%). Elemental analyses for C14H21NO3, Calcd. (%): C, 30.40; H, 4.01; N, 7.60,
45
TH-1730_126122002
Chapter 3
1 found (%): C, 30.35; H, 4.03; N, 7.75. H-NMR (600 MHz, CDCl3): δppm, 1.32 (s, 9H),
13 1.44 (s, 9H), 5.22 (s, 1H), 7.11 (s, 1H), 7.40 (s, 1H). C-NMR (150 MHz, CDCl3): δppm,
29.6, 31.6, 34.4, 35.2, 122.3, 124.8, 125.2, 136.1, 142.9, 149.7. Mass (m/z): Calcd: 251.15,
found: 250.53 (M-1).
3.4 Conclusion
In conclusion, the present work demonstrates an example of a Co(II)-salen complex, 3.1,
which reacts with NO in THF to form a stable Co(II)-nitrosyl complex, 3.2. This nitrosyl
complex reacts with KO2 to result in corresponding nitrato complex, 3.3. The reaction is
believed to proceed via formation of unstable peroxynitrite intermediate. The nitrosyl
ligand of the Co(II)-nitrosyl complex is found to be transferable. Nitrosyl transfer reaction
of this nitrosyl complex was performed with a modified tetraphenylporhyrinato Co(II)
complex, 3.4 which results in the formation of corresponding cobalt-porphyrinate nitrosyl
complex, 3.5.
3.5 References
(1) (a) Culotta, E.; Koshland, D. E. Jr. Science 1992, 258, 1862. (b) Moncada, S.;
Palmer, R. M. J.; Higgs, E. A. Pharmacol. Rev. 1991, 43, 109.
(2) (a) Furchgott, R. F. Angew. Chem. Int. Ed. 1999, 38, 1870. (b) Ignarro, L. J.; Buga,
G. M.; Wood, K. S.; Byrns, R. E.; Chaudhuri, G. Proc. Natl. Acad. Sci. U. S. A.
1987, 84, 9265. (c) Palmer, R. M. J.; Ferrige, A. G.; Moncada, S. Nature 1987, 327,
524.
(3) (a) Ignarro, L. J. Nitric Oxide: Biology and Pathobiology Ed. Academic Press, San
Diego, CA, 2000. (b) Richter-Addo, G. B.; Legzdins, P. Metal Nitrosyls Oxford
University Press, New York 1992. (c) Butler, A. R.; Williams, D. Chem. Soc. Rev.
1993, 233. (d) Feelisch, M.; Stamler, J. S. Methods in Nitric Oxide Research Ed.
46
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John Wiley and Sons, New York 1996. (e) Jia, L.; Bonaventura, C.; Bonaventura
J.; Stamler, J. S. Nature 1996, 380, 221.
(4) Fang, F. C. Nitric Oxide and Infection Ed. Kluwer Academic/Plenum Publishers,
New York 1999.
(5) (a) Ford, P. C.; Wink, D. A.; Stanbury, D. M. FEBS Lett. 1993, 326, 1. (b) Tran, N.
G.; Kalyvas, H.; Skodje, K. M.; Hayashi, T.; Moënne-Loccoz, P.; Callan, P. E.;
Shearer, J.; Kirschenbaum, L. J.; Kim, E. J. Am. Chem. Soc. 2011, 133, 1184. (c)
Goldstein, S.; Lind, J.; Merenyi, G. Chem. Rev. 2005, 105, 2457. (d) Speelman, A.
L.; Lehnert, N. Acc. Chem. Res. 2014, 47, 1106.
(6) (a) Pacher, P.; Beckman, J. S.; Liaudet, L. Physiol. Rev. 2007, 87, 315. (b)
Beckman, J. S.; Koppenol, W. H. Am. J. Physiol. 1996, 271, 1424.
(7) Radi, R. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 4003.
(8) Qiao, L; Lu, Y.; Liu, B.; Girault, H. H. J. Am. Chem. Soc. 2011, 133, 19823.
(9) Vilet, A.; Eiserich, J. P.; Halliwell, B.; Cross. C. E. J. Biol. Chem. 1997, 272, 7617.
(10) (a) Hughes, M. N.; Nicklin, H. G.; Sackrule, W. A. C. J. Chem. Soc. 1971, 3722.
(b) Babich, O. A.; Gould, E. S. Res. Chem. Intermed. 2002, 28, 575. (c) Ford, P. C.;
Lorkovic, I. M. Chem. Rev. 2002, 102, 993.
(11) (a) Herold, S.; Koppenol, W. H. Coord. Chem. Rev. 2005, 249, 499. (b) Roncaroli,
F.; Videla, M.; Slep L. D.; Olabe, J. A. Coord. Chem. Rev. 2007, 251, 1903.
(12) (a) Maiti, D.; Lee, D.-H.; Sarjeant, A. A. N.; Pau, M. Y. M.; Solomon, E. I.;
Gaoutchenova, K.; Sundermeyer, J.; Karlin, K. D. J. Am. Chem. Soc. 2008, 130,
6700. (b) Yokoyama, A.; Cho, K. B.; Karlin, K. D.; Nam, W. J. Am. Chem. Soc.
2013, 135, 14900.
(13) Kumar, P.; Lee, Y.-M.; Park, Y. J.; Siegler, M. A.; Karlin, K. D.; Nam, W. J. Am.
Chem. Soc. 2015, 137, 4284.
47
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(14) Ungermann, C. B.; Caulton, K. G. J. Am. Chem. Soc. 1976, 98, 3862.
(15) Franz, K. J.; Lippard, S. J. Inorg. Chem. 2000, 39, 3722.
(16) Kumar, P.; Lee, Y.-M.; Hu, L.; Chen, J.; Park, Y. J.; Yao, J.; Chen, H.; Karlin, K.
D.; Nam, W. J. Am. Chem. Soc. 2016, 138, 7753.
(17) (a) Clarke, R. M.; Hazin, K.; Thompson, J. R.; Savard, D.; Prosser, K. E.; Storr, T.
Inorg. Chem. 2016, 55, 762. (b) Deiasi, R.; Holt, S. L.; Post, A. Inorg. Chem. 1971,
10, 1498.
(18) Richter-Addo, G. B.; Hodge, S. J.; Yi, G.-B.; Khan, M. A.; Ma, T.; Caemelbeck, E.
V.; Huo, N.; Kadish, K. M. Inorg. Chem. 1996, 35, 6530.
(19) (a) Enemark, J. H.; Feltham, R. D. Coord. Chem. Rev. 1974, 5, 686. (b) Richter-
Addo, G. B.; Legzdins, P. Metal Nitrosyls, Oxford University Press: New York,
1992. (c) McCleverty, J. A. Chem. Rev. 2004, 104, 403. (d) Berto, T. C.; Speelman,
A. L.; Zheng, S.; Lehnert, N. Coord. Chem. Rev. 2013, 257, 244.
(20) (a) Clarkson, S. G.; Basolo, F. J. Chem. Soc. Chem. Commun. 1972, 119, 670. (b)
Clarkson, S. G.; Basolo, F. Inorg. Chem. 1973, 12, 1528. (c) Subedi, H.; Brasch, N.
E. Inorg. Chem. 2013, 52, 11608. (d) Frech, C. M.; Blacque, O.; Schmalle, H. W.;
Berke, H. Dalton Trans. 2006, 4590.
(21) Kalita, A.; Deka, R. C.; Mondal, B. Inorg. Chem. 2013, 52, 10897.
(22) Kalita, A.; Kumar, P.; Mondal, B. Chem. Commun. 2012, 48, 4636.
(23) SMART, SAINT and XPREP, Siemens Analytical X-ray Instruments Inc.,
Madison, Wisconsin, USA 1995.
(24) Sheldrick, G. M.; SADABS: University of Gottingen, Germany 1999.
(25) Sheldrick, G. M. SHELXS-97, University of Gottingen, Germany 1997.
(26) Farrugia, L. J. ORTEP-3 for Windows - a Version of ORTEP-III with a Graphical
User Interface (GUI). J. Appl. Crystallogr. 1997, 30, 565.
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(27) Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.; Jarjayes, O.; Vezin,
H.; Luneau, D.; Orio, M.; Thomas, F. Inorg. Chem. 2012, 51, 10557.
(28) (a) Adler, A. D.; Longo, F. R.; Finarelli.; Goldmacher, J.; Assour, J.; Korsakoff, L.
J. Org. Chem. 1967, 32, 476. (b) Kadish, K. M.; Smith, K. M.; Guilard, R. The
Porphyrin Handbook Ed. Elsevier 1999.
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TH-1730_126122002 Chapter 4
Reaction of a Nitrosyl Complex of Cobalt Porphyrin with Hydrogen Peroxide: Putative Formation of Peroxynitrite Intermediate
Abstract
A cobalt porphyrin complex, [(L4)Co], 4.1 {L4 = 5,10,15,20-tetrakis(4'-
chlorophenyl)porphyrinate dianion} in dichloromethane solution was subjected to react
with nitric oxide (NO) gas and resulted in the formation of the corresponding nitrosyl
complex, [(L4)Co(NO)], 4.2 having {CoNO}8 description. It was characterized by
spectroscopic studies and single crystal X-ray structure determination. It did not react with
dioxygen. However, in CH2Cl2/CH3CN solution, it reacted with H2O2 to result in the Co-
nitrito complex, [(L4)Co(NO2)], 4.3 with the simultaneous release of O2. It induced ring
nitration to the added phenol in an appreciable yield. The reaction presumably proceeds
through the formation of corresponding Co-peroxynitrite intermediate.
TH-1730_126122002
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4.1 Introduction
Nitric oxide (NO) is known to play the key roles in many physiological processes like
neurotransmission, vasodilation, immune cytotoxicity, etc.1,2 It can also behave as a
cytotoxic effector and /or pathogenic mediator when produced at high concentrations.3 It is
believed that the cytotoxicity of NO is due to the formation of secondary reactive nitrogen
- 3,4 species (RNS) like peroxynitrite (ONOO ) or nitrogen dioxide (NO2). The formation of
these secondary RNS may result from the oxidation of NO in the presence of oxidants like
- 3 superoxide radicals (O2 ), hydrogen peroxide (H2O2) and/or transition metal ions.
However, nitric oxide deoxygenases (NODs) are known to maintain the in vivo level of
NO.5,6 It is known that the oxy-heme (i.e. iron(III)-superoxo) species of the NODs react
- with NO to result in nitrate (NO3 ) ion which is biologically benign. This reaction is
believed to proceed through the formation of ONOO- intermediates. The examples of
discrete metal-peroxynitrite complexes are rare; only a cobalt-peroxynitrite complex is
known to be discretely characterized.7 However, there are number of examples where
metal-peroxynitrites are proposed to form as transient intermediate in the reaction of metal-
oxygen species with NO. In those cases, metal-superoxo complexes of heme and non-heme
iron, cobalt and copper were reported to react with NO to result in metal-peroxynitrite
intermediates.8,9 Apart from those, recently the reaction of a non-heme Cr(IV)-peroxo
complex with NO was reported to give Cr(III)-nitrate; whereas Cr(III)-superoxo has been
shown to result in corresponding Cr(IV)-oxo and NO2 through the formation of a presumed
Cr(III)-peroxynitrite intermediate.10
On the other hand, a few instances involving the reaction of metal-nitrosyl and O2 to form
transient metal-peroxynitrite have been exemplified. For instance, Clarkson and Basolo
reported the reaction of cobalt-nitrosyl complex with O2 which resulted in the formation of
corresponding nitrite product.11 Kim et. al reported the reaction of a non-heme
51
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Chapter 4
12 10 dinitrosyliron with O2 to afford nitrate. In a similar reaction {CuNO} species was found
13 10 to result in corresponding nitrite and O2. We reported the reaction of {CuNO} complex
I with H2O2 to give copper-nitrato complex via thermal decomposition of a presumed Cu -
peroxynitrite intermediate.14 Thus, the metal complex mediated reaction of NO with
various reduced O2 species is of immense importance for the understanding of NO
reactivity in biological systems. In addition, peroxynitrite mediated nitration of added
phenols resembles the biologically well-known tyrosine nitration.
Herein we describe the synthesis, structural characterization of a nitrosyl complex of cobalt
porphyrin, [(L4)Co(NO)]. In the presence of hydrogen peroxide (H2O2) the cobalt-nitrosyl
complex exhibits NOD reactivity where the intermediate formation of peroxynitrite is
implicated.
4.2 Results and discussion
The cobalt porphyrin complex, [(L4)Co], 4.1 [L4 = (5,10,15,20-tetrakis(4'-
chlorophenyl)porphyrinate dianion] was prepared following an earlier reported
procedure.15 Elemental and spectral analyses confirmed the formation of the complex
(Experimental section and Appendix III). Bubbling of an excess of NO gas to the degassed
dicholoromethane solution of 4.1 resulted in the formation of corresponding Co-nitrosyl
complex [(L4)Co(NO)], 4.2 (Experimental section and Scheme 4.1). It was isolated as
solid and characterized spectroscopically as well as by single crystal X-ray structure
determination. The ORTEP diagram of 4.1 is shown in figure 4.1a. The crystallographic
data and metric parameters are listed in the appendix III. The crystal structure revealed that
the cobalt center is six coordinated with a distorted octahedral geometry. The four N-atoms
from the porphyrinate dianion bind the metal ion to form a square plane and two THF units
are bonded to the metal centre from the axial position. The average Co-Np bond distance
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TH-1730_126122002
Chapter 4
was 1.989(2) Å which is within the range observed for hexa-coordinated Co(II)-
16a porphyrinates. Co-OTHF distances were found to be 2.404 (3) Å. The average Co-OTHF
16b distance of 2.204 (2) Å was reported in case of [(F28TPP)Co(THF)2]. In case of five
coordinated Co(II)-porphyrinate with THF as an axial ligand, this distance was reported to
be 2.206 (2) Å.16a It is to be noted that for six-coordinated Co(III)-porphyrinates, the
16d 1 average Co-Np distance lies in the range of 1.950 Å. In H-NMR spectrum, complex 4.1
in CDCl3 displayed peak broadening expected for low spin cobalt(II) porphyrinates
(Appendix III). In the UV-visible spectrum, 4.2 exhibited characteristic absorption bands
at 415 nm (ε/M-1cm-1, 2.21 × 105) and 538 nm (ε/M-1cm-1, 3.1 × 104) (Figure 4.2a).
Scheme 4.1. Synthesis of nitrosyl complex 4.2.
Complex 4.2 was silent in the X-band EPR spectroscopy suggesting the presence of
Co(III) centre having [CoIII-NO-] or {CoNO}8 description according to the Enemark-
Feltham notation.17 In 1H-NMR spectrum, the pyrrole protons appeared as singlet at 8.94
and 8.82 ppm and the protons from the phenyl group appeared as doublet at 8.05 and 7.64
ppm (Appendix III). In the FT-IR spectrum, the NO stretching band appeared at 1701 cm-1
(Figure 4.2b) which is consistent with the {CoNO}8 description. This frequency is slightly
higher than those reported earlier for [(TPP)Co(NO)] (1689 cm-1, in KBr) and
-1 18 [(OEP)Co(NO)] (1675 cm , in CH2Cl2). In case of cobalt porphyrin nitrosyls of the form
[{T(p/m-X)PP}Co(NO)] [p/m-X = p-OCH3, p-CH3, m-CH3, p-H, m-OCH3, p-OCF3, p-CF3,
p-CN etc.], the ῡNO band in CH2Cl2 was reported previously to appear in the range of 1681-
1695 cm-1.22
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TH-1730_126122002
Chapter 4
(a) (b)
Figure 4.1. ORTEP diagram of complexes (a) 4.1 (b) 4.2. (30% thermal ellipsoid plot. H-atoms and solvent molecules are not shown for clarity).
(a) (b)
Figure 4.2. (a) UV-visible spectra of complexes 4.1 (black) and 4.2 (red) in dichloromethane. (b) FT-IR spectrum of complex 4.2 in KBr.
The ESI mass spectrum of 4.2 was dominated by the peak at m/z, 809.05 assignable to
[(L4)Co]+ [calcd. m/z, 809.00] . This is attributed to the facile loss of axial NO ligand
under experimental condition.19
Furthermore, complex 4.2 was characterized by a single crystal X-ray structure
determination. The ORTEP diagram is shown in figure 4.1b. The crystallographic data and
other metric parameters are listed in the appendix III. The structure revealed that the NO is
coordinated axially to the cobalt centre in an end-on fashion resulting in a distorted square-
pyramidal geometry around the cobalt centre. Notably, the Co-Np distances in 4.2 are in
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TH-1730_126122002
Chapter 4
the range of 1.974 (6) Å to 1.990 (6) Å with an average of 1.981 (6) Å. This distance is bit
larger than that reported for Co(III)-porphyrinates; but in the range of Co(II)-
16 porphyrinates. The Co-NO moiety is bent with a bond angle of 118.4. The Co-NNO bond
distance is 1.846 (7) Å. The N-O distance was found to be 1.230 (1) Å. These are well
within the range of those reported previously.19 The observation of bent NO is consistent
with {CoNO}8 description.
Reactivity of complex 4.2 with O2 and H2O2
The reactivity of 4.2 with oxidants like O2 and H2O2 have been explored. It was observed
that 4.2 in CH2Cl2 solution is inert toward O2. This is indeed in accord with the earlier
reported observations that five coordinated square pyramidal cobalt(II) nitrosyls do not
20 react with O2. Similar observation was reported with Co(III)-nitrosyls of 12 and 13-
20 membered N-tetramethylated cyclam ligands as well. However, in CH2Cl2/CH3CN
solution 4.2 was found to react with H2O2. Addition of H2O2 to the CH2Cl2/CH3CN
solution of 4.2 at -40 C gave corresponding Co-nitrito complex, 4.3 (Scheme 4.2).
Scheme 4.2. Reaction of complex 4.2 with H2O2.
In the UV-visible spectrum, the characteristic absorption bands of 4.2 at 415 nm and 538
nm disappeared with the formation of new ones at 430 nm (ε/M-1cm-1, 2.84 × 105), 546
(ε/M-1 cm-1, 3.3 × 104) and 665 nm (ε/M-1cm-1, 4.2 × 103), respectively (Figure 4.3a).
Spectral titration suggested that the stoichiometric ratio of 4.2 with H2O2 is 1:1. Complex
4.3 was isolated and characterized as the corresponding Co(III)-nitrito complex,
55
TH-1730_126122002
Chapter 4
[(L4)Co(NO2)] on the basis of microanalysis and spectroscopic analyses (Experimental
section and Appendix III).
The appearance of the signals in the diamagnetic range in 1H-NMR spectrum of 4.3 in
CDCl3 is in agreement with the existence of low spin cobalt(III) centre in the complex. The
pyrrole protons appeared together at 8.90 ppm; whereas the phenyl protons appeared as
two doublets at 8.09 and 7.73 ppm, respectively. The ESI mass spectrum of 4.3 displayed
the molecular ion peak (M+1) at m/z, 856.08 (calcd. 854.99) (Figure 4.3b). The observed
isotopic distribution pattern is also in good agreement with the simulated one.
(a) (b)
Figure 4.3. UV-visible spectral monitoring of complex 4.2 (black) and after addition of H2O2 to result in complex 4.3 (red) at -40 C. (b) ESI-mass spectrum of complex 4.3 in acetonitrile. Inset: (i) experimental (black) and (ii) simulated (red) isotopic distribution pattern.
In the FT-IR spectrum, the stretching frequencies at 1270 cm-1 is assignable to the nitrite
- (NO2 ) stretching (Appendix III). In cases of earlier reported cobalt porphyrin nitrites, this
stretching was observed in the similar range.21 Comparing the other reported results of
nitrites of metal porphyrinates, it is logical to assume that 4.3 as an N-bound nitrite.21 Even
after several attempts, we could not get the X-ray quality crystal of 4.3 for structural
characterization. The formation of cobalt-nitrito complex is supposed to be accompanied
by the release of O2 (Scheme 4.2). The formation of O2 in the reaction was confirmed by
56
TH-1730_126122002
Chapter 4
its detection in the GC-mass spectral analysis of the headspace gas of the reaction flask and
by alkaline pyrogallol test, as well. However, no attempts were made to quantify the
amount of O2 formed in the reaction.
It is to be noted that in case of aqueous peroxynitrite chemistry, it is reported earlier that
- - + 22 ONOOH and ONOO react to result in NO2 , O2 and H . On the other hand, Cu-
peroxynitrite complexes are also known to undergo transformation to Cu(II)-nitrito
13 complexes with the release of O2. Though these observations suggested the formation of
Co-peroxynitrite intermediate, no hint of it was observed in UV-visible spectroscopic
monitoring even at -80 C.
While we do not have direct spectroscopic evidence of formation of Co-peroxynitrite
intermediate, we sought chemical evidence for the postulated species. When the reaction of
4.2 with H2O2 is followed by the addition of 2,4-di-tert-butylphenol (DTBP), complex 4.3
is formed and unreacted DTBP is recovered. However, effective nitration (ca. 50%) is
observed when DTBP is added before the addition of one equivalent H2O2 (Scheme 4.3).
Work-up of the reaction mixture reveals the presence of corresponding Co(III)-hydroxo
product, [(L4)Co(OH)], 4.4 (ca. 70% ) and 2,4-di-tert-butyl-6-nitrophenol (ca. 50%). A
product from oxidative coupled phenol is also observed though in very low yield (ca.
10%). It would be worth to mention here that the phenol nitration has been extensively
used to provide evidence in support of presence of metal-peroxynitrite intermediates.
Scheme 4.3. Phenol ring nitration by complex 4.2 in the presence of H2O2.
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Chapter 4
4.3 Experimental section
4.3.1 Materials and methods
All reagents and solvents were purchased from commercial sources and were of reagent
grade. Acetonitrile was distilled from calcium hydride. Deoxygenation of the solvent and
solutions were effected by repeated vacuum/purge cycles or bubbling with argon for 30
minutes. NO gas was purified by passing through KOH and P2O5 column. UV-visible
spectra were recorded on a Agilent Cary 8454 UV-visible spectrophotometer. FT-IR
spectra were taken on a Perkin Elmer spectrophotometer with samples prepared as KBr
pellets. 1H-NMR spectra were obtained with a 600 MHz Varian FT spectrometer.
Chemical shifts (ppm) were referenced either with an internal standard (Me4Si) for organic
compounds or to the residual solvent peaks. Elemental analyses were obtained from a
Perkin Elmer Series II Analyzer. Mass spectra were recorded on a Waters, Model: Q-Tof
Premier instrument with ESI mode of ionization.
The single crystal of the ligand was obtained by slow evaporation from a CHCl3 solution;
whereas the crystals of 4.1 and 4.2 were grown from the respective THF solutions. The
intensity data were collected using a Bruker SMART APEX-II CCD diffractometer,
equipped with a fine focus 1.75 kW sealed tube MoK radiation ( = 0.71073 Å) at 273(3)
K, with increasing (width of 0.3° per frame) at a scan speed of 3 s/frame. The SMART
software was used for data acquisition. Data integration and reduction were undertaken
with SAINT and XPREP software.23 Multi-scan empirical absorption corrections were
applied to the data using the program SADABS.24 Structures were solved by direct
methods using SHELXS-2014 and refined with full-matrix least squares on F2 using
SHELXL-2014/7.25 Structural illustrations have been drawn with ORTEP-3 for
Windows.26
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Chapter 4
4.3.2 Syntheses
4.3.2.1 Synthesis of ligand L4H2, [5,10,15,20-tetrakis(4'-chlorophenyl)porphyrin]
The ligand L4H2 was prepared by following earlier reported general procedure of
porphyrin synthesis with slight modification. p-chlorobenzaldehyde (2.80 g, 20 mmol) and
freshly distilled pyrrole (1.34 g, 20 mmol) were added to 37 mL of propionic acid. After
refluxing for 2 h the solution was cooled to room temperature. It was then neutralized by
aqueous Na2CO3. A precipitate appeared and it was filtered off. The crude mass was
subjected to purification using column chromatography using CH2Cl2 and hexane as
solvent to afford the desired ligand as purple solid. Yield: 0.750 g (20%). Elemental
analyses for C44H26N4Cl4. Calcd. (%): C, 70.23; H, 3.48; N, 7.45, found (%): C, 70.15; H,
3.47; N, 7.58. FT-IR (in KBr): 3444, 2922, 1487, 1469, 1090, 1015, 966, 797, 728, 491
-1 1 cm . H-NMR (600 MHz, CDCl3): δppm, 7.75-7.74 (d, 8H), 8.14-8.13(d, 8H), 8.85 (s, 8H).
Mass (m/z): Calcd: 752.52, found: 753.08 (M+1).
4.3.2.2 Syntheses of complexes
(a) 4.1, [(L4)Co]
To a mixture of ligand, L4H2 (0.188 g, 0.25 mmol) in CHCl3 (12 mL) and acetic acid (12
mL), cobalt(II) acetate tetrahydrate (0.622 g, 2.50 mmol) was added. After refluxing at
120 °C for 2 h, the precipitate was filtered and washed with methanol. The crude mass was
subjected to column chromatography using silica gel column and CHCl3: hexane (2:3, v/v)
solvent mixture to result in complex 4.1 as violet solid. Yield: 0.141 g (70%). Elemental
analyses for C44H24N4Cl4Co, Calcd. (%): C, 65.29; H, 2.99; N, 6.92, found (%): C, 65.22;
-1 -1 5 -1 H, 2.98; N, 7.02. UV-visible (CH2Cl2): 410 nm (ε/M cm , 2.24 × 10 ), 525 nm (ε/M
cm-1, 2.8 × 104). FT-IR (in KBr): 1541, 1486, 1348, 1090, 1001, 795, 719, 505 cm-1. Mass
(m/z): Calcd. 809.00, found: 809.05 (molecular ion peak).
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Chapter 4
(b) 4.2, [(L4)Co(NO)]
To a dry and degassed CH2Cl2 solution of 4.1 (0.8 g, 1 mmol), NO gas was bubbled for 5
min. The violet color of the solution changed to dark red. The excess of NO gas was
removed by applying several cycles of vacuum/argon purge. The crude product was
isolated by drying under reduced pressure. The pure crystalline complex 4.2 was obtained
after recrystallization from THF. Yield: 0.680 g (80%). Elemental analyses for
C44H24N5OCl4Co, Calcd. (%): C, 62.96; H, 2.88; N, 8.34, found (%): C, 62.92; H, 2.85; N,
-1 -1 5 -1 -1 4 8.41. UV-visible (CH2Cl2): 415 nm (ε/M cm , 2.21 × 10 ), 538 nm (ε/M cm , 3.1 × 10 ).
FT-IR (in KBr): 1701, 1485, 1346, 1090, 999, 795, 714, 504 cm-1. 1H-NMR (600 MHz,
CDCl3): δppm, 7.64-7.66 (d, 8H), 8.05-8.00 (d, 8H), 8.82 (s, 4H), 8.94 (s, 4H).
(c) 4.3, [(L4)Co(NO2)]
Complex 4.2 (0.418 g, 0.5 mmol) was dissolved in 20 mL of dried and degassed
CH2Cl2/CH3CN (v/v, 5:1) and the solution was cooled at -40 C. To this cold solution, pre-
cooled hydrogen peroxide (37% v/v, 200 µL) was added and stirred for 1/2 h. The red
color of the solution changed to yellowish red. The solution was then warmed to room
temperature and dried under reduced pressure. Yield: 0.254 g (60%). Elemental analyses
for C44H24N5O2Cl4Co, Calcd. (%): C, 61.78; H, 2.83; N, 8.19, found (%): C, 61.72; H,
-1 -1 5 2.80; N, 8.25. UV-visible (CH2Cl2/CH3CN): 430 nm (ε/M cm , 2.84 × 10 ), 546 (ε/
M-1cm-1, 3.3 × 104), 665 nm (ε/M-1cm-1, 4.2 × 103). FT-IR (in KBr): 1637, 1485, 1390,
-1 1 1321, 1270, 1090, 1063, 798, 504 cm . H-NMR (600 MHz, CDCl3): δppm, 7.71-7.73 (d,
8H), 8.07-8.09 (d, 8H), 8.90 (s, 8H). Mass (m/z): Calcd: 854.99, found: 856.08 (M+1).
(d) 4.4 and 2,4-di-tert-butyl-6-nitrophenol
Complex 4.2 (0.837 g, 1 mmol) was dissolved in 20 mL of dried and degassed
CH2Cl2/CH3CN (v/v, 5:1) and the solution was cooled at -40 C. To this cold solution, pre-
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cooled 2,4-di-tert-butylphenol (1.03 g; 5 mmol) was added and stirred for 5 min. To this,
pre-cooled H2O2 (37% v/v, 400 µL) in acetonitrile was added and the resulting mixture
was stirred for 30 min at -40 °C. The reaction mixture was then warmed to room
temperature and dried under reduce pressure. The solid mass was then subjected to column
chromatography using silica gel column and hexane as solvent to obtain 2,4-di-tert-butyl-
6-nitrophenol. Yield: 0.125 g (50%) and complex 4.4 as solid. Yield: 0.575 g (70%).
Complex 4.4: Elemental analyses for C44H25N4OCl4Co, Calcd. (%): C, 63.95; H, 3.05; N,
6.78, found (%): C, 63.91; H, 3.07; N, 6.86. FT-IR (in KBr): 3439, 2958, 1656, 1491,
-1 -1 -1 5 1369, 1090, 998, 785 cm . UV-visible (CH2Cl2/CH3CN): 390 nm (ε/M cm , 1.95 × 10 ),
498 nm (ε/M-1cm-1, 2.65 × 104), 621 nm (ε/M-1cm-1, 2.58 × 102).
4.4 Conclusion
In conclusion, a nitrosyl complex of cobalt porphyrin, [(L4)Co(NO)] having {CoNO}8
description was synthesized and characterized structurally. It was found to be inert toward
8 dioxygen. However, in CH2Cl2/CH3CN solution, the {CoNO} complex reacts with H2O2
to result in the corresponding Co-nitrito complex, [(L4)Co(NO2)] with the release of O2.
The reaction presumably proceeds through the formation of Co-peroxynitrite intermediate.
Thus, the present study demonstrates an example of the reaction of a nitrosyl complex of
metal-porphyrin with H2O2 which led to the formation of the corresponding nitrito
complex along with the release of O2 via a presumed metal-peroxynitrite intermediate.
4.5 References
(1) Ignarro L. J. Nitric oxide: Biology and Pathobiology Ed. Academic press; San
Diego, 2000.
(2) (a) Moncada, S.; Palmer, R. M. J.; Higgs, E. A. Pharmacol. Rev. 1991, 43, 109. (b)
Butler, A. R.; Williams, D. L. H. Chem. Soc. Rev. 1993, 22, 233. (c) Feelisch, M.;
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Stamler, J. S. Methods in Nitric Oxide Research Ed. John Wiley and Sons, New
York 1996. (d) Jia, L.; Bonaventura, C.; Bonaventura, J.; Stamler, J. S. Nature
1996, 380, 221. (e) Galdwin, M. T.; Lancaster, J. R. Jr.; Freeman, B. A.;
Schechter, A. N. Nat. Med. 2003, 9, 496.
(3) (a) Radi, R. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 4003. (b) Qiao, L.; Lu, Y.;
Liu, B.; Girault, H. H. J. Am. Chem. Soc. 2011, 133, 19823. (c) Ford, P. C.; Wink,
D. A.; Stanbury, D. M. FEBS Lett. 1993, 326, 1. (d) Tran, N. G.; Kalyvas, H.;
Skodje, K. M.; Hayashi, T.; Moënne-Loccoz, P.; Callan, P. E.; Shearer, J.;
Kirschenbaum, L. J.; Kim, E. J. Am. Chem. Soc. 2011, 133, 1184.
(4) (a) Goldstein, S.; Lind, J.; Merenyi, G. Chem. Rev. 2005, 105, 2457. (b) Pacher, P.;
Beckman, J. S.; Liaudet, L. Physiol. Rev. 2007, 87, 315. (c) Blough, N. V.;
Zafiriou, O. C. Inorg. Chem. 1985, 24, 3502. (d) Nauser, T.; Koppenol, W. H. J.
Phys. Chem. A 2002, 106, 4084. (e) Speelman, A. L.; Lehnert, N. Acc. Chem. Res.
2014, 47, 1106. (f) Fry, N. L.; Mascharak, P. K. Acc. Chem. Res. 2011, 44, 289.
(5) (a) Doyle, M. P.; Hoekstra, J. W. Inorg. Biochem. 1981, 14, 351. (b) Cooper, C. E.;
Torres, J.; Sharpe, M. A.; Wilson, M. T. FEBS Lett. 1997, 414, 281. (c) Tocheva,
E. I.; Rosell, F. I.; Mauk, A. G.; Murphy, M. E. Science 2004, 304, 867.
(6) (a) Gardner, P. R.; Gardner, A. M.; Martin, L. A.; Salzman, A. L. Proc. Natl. Acad.
Sci. U. S. A. 1998, 95, 10378. (b) Ford, P. C.; Lorkovic, I. M. Chem. Rev. 2002,
102, 993. (c) Schopfer, M. P.; Mondal, B.; Lee, D.-H.; Sarjeant, A. A. N.; Karlin,
K. D. J. Am. Chem. Soc. 2009, 131, 11304.
(7) Wick, P. K.; Kissner, R.; Koppenol, W. H. Helv. Chim. Acta 2000, 83, 748.
(8) (a) Roncaroli, F.; Videla, M.; Slep, L. D.; Olabe, J. A. Coord. Chem. Rev. 2007,
251, 1903. (b) Maiti, D.; Lee, D.-H.; Sarjeant, A. A. N.; Pau, M. Y. M.; Solomon,
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E. I.; Gaoutchenova, K.; Sundermeyer, J.; Karlin, K. D. J. Am. Chem. Soc. 2008,
130, 6700.
(9) (a) Goodwin, J. A.; Coor, J. L.; Kavanagh, D. F.; Sabbagh, M.; Howard, J. W.;
Adamec, J. R.; Parmley, D. J.; Tarsis, E. M.; Kurtikyan, T. S.; Hovhannisyan, A.
A.; Desrochers, P. J.; Standard, J. M. Inorg. Chem. 2008, 47, 7852. (b) Kurtikyan,
T. S.; Ford, P. C. Chem. Commun. 2010, 46, 8570. (c) Kurtikyan, T. S.; Eksuzyan,
S. R.; Goodwin, J. A.; Hovhannisyan, G. S. Inorg. Chem. 2013, 52, 12046.
(10) (a) Yokoyama, A.; Han, J. E.; Cho, J.; Kubo, M.; Ogura, T.; Siegler, M. A.; Karlin,
K. D.; Nam, W. J. Am. Chem. Soc. 2012, 134, 15269. (b) Yokoyama, A.; Cho, K.
–B.; Karlin, K. D.; Nam, W. J. Am. Chem. Soc. 2013, 135, 14900.
(11) (a) Clarkson, S. G.; Basolo F. J. Chem. Soc. Chem. Commun. 1972, 119, 670. (b)
Clarkson, S. G.; Basolo F. Inorg. Chem. 1973, 12, 1528.
(12) Skodje, K. M.; Williard, P. G.; Kim, E. Dalton Trans. 2012, 41, 7849.
(13) Park, G. Y.; Deepalatha, S.; Puiu, S. C.; Lee, D.-H.; Mondal, B.; Sarjeant, A. A. N.;
del Rio, D.; Pau, M. Y. M.; Solomon, E. I.; Karlin, K. D. J. Biol. Inorg. Chem.
2009, 14, 1301.
(14) (a) Kalita, A.; Kumar, P.; Mondal, B. Chem. Commun. 2012, 48, 4636. (b) Kalita,
A.; Deka, R. C.; Mondal, B. Inorg. Chem. 2013, 52, 10897.
(15) (a) Li, C.; Wang, Q.; Shen, B.; Xiong, Z.; Chen, C. J. Chem. Eng. Data 2014, 59,
3953.(b) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976, 98,
3484.
(16) (a) Dey, S.; Rath, S. P. Dalton Trans. 2014, 43, 2301. (b) Smirnov, V. V.; Woller,
E. K.; DiMagno, S. G. Inorg. Chem. 1998, 37, 4971. (c) Cheng, R. J.; Chen, Y. H.;
Chen, C. C.; Lee, G. H.; Peng, S. M.; Chen, P. P. Y. Inorg. Chem. 2008, 53, 8848.
(d) Dey, S.; Sil, D.; Rath, S. P. Angew. Chem. Int. Ed. 2016, 55, 996.
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(17) (a) Enemark, J. H.; Feltham, R. D. Coord. Chem. Rev. 1974, 13, 339. (b) Richter-
Addo, G. B.; Legzdins, P. Metal Nitrosyls: Oxford University Press: New York
1992. (c) McCleverty, J. A. Chem. Rev. 2004, 104, 403. (d) Berto, T. C.; Speelman,
A. L.; Zheng, S.; Lehnert, N. Coord. Chem. Rev. 2013, 257, 244.
(18) (a) Scheidt, W. R.; Hoard, J. L. J. Am. Chem. Soc. 1973, 95, 8281. (b) Wayland, B.
B.; Minkiewicz, J. V.; Abd-Elmageed, M. E. J. Am. Chem. Soc. 1974, 96, 2795. (c)
Fujita, E.; Chang, C. K.; Fazer, J. J. Am. Chem. Soc. 1985, 107, 7665. (d) Fujita, E.;
Fazer, J. J. Am. Chem. Soc. 1983, 105, 6743.
(19) Addo, G. B. R.; Hodge, S. J.; Yi, G. B.; Khan, M. A.; Ma, T.; Caemelbecke, E. V.;
Guo, N.; Kadish, K. M. Inorg. Chem. 1996, 35, 6530.
(20) Kumar, P.; Lee, Y.-M.; Park, Y. J.; Siegler, M. A.; Karlin, K. D.; Nam, W. J. Am.
Chem. Soc. 2015, 137, 4284.
(21) (a) Goodwin, J.; Kurtikyan, T.; Standard, J.; Walsh, R.; Zheng, B.; Parmley, D.;
Howard, J.; Green, S.; Mardyukov, A.; Przybyla, D. E. Inorg. Chem. 2005, 44,
2215. (b) Wyllie, G. R. A.; Scheidt, W. R. Chem. Rev. 2002, 102, 1067.
(22) (a) Pfeiffer, S.; Gorren, A. C. F.; Schmidt, K.; Werner, E. R.; Hansert, B.; Bohle,
D. S.; Mayer, B. J. Biol. Chem. 1997, 272, 3465. (b) Coddington, J. W.; Hurst, J.
K.; Lymar, S. V. J. Am. Chem. Soc. 1999, 121, 2438. (c) Koppenol, W. H.; Bounds,
P. L.; Nauser, T.; Kissner, R.; Rüegger, H. Dalton Trans. 2012, 41, 13779.
(23) SMART, SAINT and XPREP, Siemens Analytical X-ray Instruments Inc., Madison,
Wisconsin, U. S. A. 1995.
(24) Sheldrick, G. M. SADABS: software for Empirical Absorption Correction,
University of Gottingen, Institut fur Anorganische Chemieder Universitat,
Tammanstrasse 4, D-3400 Gottingen, Germany 1999.
(25) Sheldrick, G. M. SHELXS-2014, University of Gottingen, Germany.
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(26) Farrugia, L. J. ORTEP-3 for Windows - a version of ORTEP-III with a Graphical
User Interface (GUI) J. Appl. Crystallogr. 1997, 30, 565.
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Reaction of a Co(III)-Peroxo Complex and NO: Formation of a Putative Peroxynitrite Intermediate
Abstract
The complex, [(L1)2Co]Cl2, 2.1 of the ligand L1 {L1 = bis(2-ethyl-4-methylimidazol-5-
yl)methane} upon reaction with H2O2 in methanol solution at -40 °C resulted in the
+ formation of the corresponding Co(III)-peroxo complex, [(L1)2Co(O2)] , 5.1. The addition
of NO gas to the freshly generated solution of the complex 5.1 led to the formation of the
Co(II)-nitrato complex, 5.2 through the putative formation of a Co(II)-peroxynitrite
intermediate, 5.1a. The intermediate 5.1a was found to mediate the nitration of the
externally added phenol resembling the nitration of tyrosine in biological systems.
TH-1730_126122002
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5.1 Introduction
Nitric oxide (NO) is an important small molecule that is involved in various physiological
processes such as neurotransmission, vasodilation, immune response, etc.1-4 Under normal
conditions, NO is produced by the nitric oxide synthases (NOS) at low concentration.5
However, an excess production of NO can have detrimental effects via the formation of
secondary reactive nitrogen species (RNS) such as nitrogen dioxide (NO2) and
peroxynitrite (ONOO-).6 Both of these RNS are known to play important roles in bio-
molecule oxidation leading to the oxidative and nitrosative stress.7 The nitric oxide
dioxygenases (NODs) are known to control the in vivo level of NO by converting it into
- 8 the biologically benign nitrate (NO3 ) and thereby removing the excess of it. The
- conversion of NO to NO3 by NODs is believed to proceed through the formation of a
peroxynitrite intermediate.8,9 Peroxynitrite is proposed to form in vivo in a diffusion
- - controlled reaction of NO and O2 anion or H2O2 and nitrite (NO2 ) in the presence of the
peroxidase enzymes.10,11 An aqueous peroxynitrite is known to isomerize to the nitrate
- 8 (NO3 ) very easily. In the literature, peroxynitrites are exemplified to form either in the
III _ - reaction of oxy-heme (formally, Fe O2 ) proteins with NO or metal-nitrosyls with O2 or
- 9,12 O2 . Recently, the peroxynitrite intermediates have been shown to from in the reaction
of NO with Cr–superoxo or peroxo species. A Cr(IV)-peroxo complex {[(12-
+ TMC)Cr(O2)(Cl)] } {12-TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane}
is reported to react with NO to result in a Cr(III)-nitrato complex, {[(12-
+ TMC)Cr(NO3)(Cl)] }; whereas the reaction of Cr(III)-superoxo complex, {[(14-
+ TMC)Cr(O2)(Cl)] } and NO resulted in the corresponding Cr(IV)-oxo complex, {[(14-
+ TMC)Cr(O)(Cl)] ) and NO2 presumably via the formation of a Cr(III)-peroxynitrite
intermediate, [(14-TMC)Cr(OONO)(Cl)]+.13-15 Nam et al. reported the reactivity of an
+ + Fe(III)-peroxo complex, [(14-TMC)Fe(O2)] with NO as an approach for generating the
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peroxynitrite intermediate which led to the formation of an Fe(III)-nitrato complex, [(14-
+ 16 TMC)Fe(NO3)(F)] . On the other hand, Basalo et al. reported the reaction of a Co-
nitrosyl with O2 to result in the Co-nitrite where the intermediate formation of a
peroxynitrite is presumed.17 Similarly, a non-heme dinitrosyliron complex has been shown
18 to result in the corresponding nitrite in its reaction with O2. Recently, Karlin and
coworkers reported the reaction of a mixed-valent nitrosyl complex of Cu(I)Cu(II),
- 2+ [Cu2(UN-O )(NO)] with O2 to result in the superoxide and nitrosyl adduct, [Cu2(UN-
- - 2+ O )(NO)(O2 )] which was subsequently converted to the corresponding peroxynitrite
complex.19
- In our laboratory, we have been working on the reactivity of the metal nitrosyls with O2
10 anion and H2O2 with an aim to generate a peroxynitrite species. For example, {CuNO}
complex of the bis(2-ethyl-4-methylimidazol-5-yl)methane ligand was found to react with
H2O2 to afford the corresponding Cu(I)-nitrite and the formation of a Cu(I)-peroxynitrite
20 - intermediate is presumed. The same nitrosyl complex, upon reaction with O2 anion was
observed to result in Cu(II)-nitrite.20 In an another example, the {CuNO}10 complex in the
presence of H2O2 was shown to form Cu(II)-nitrite via a presumed peroxynitrite
intermediate.21 Cobalt-nitrosyls, both with porphyrin and non-porphyrin ligands, were
- 17,22 reported to react with O2 and O2 to afford cobalt-nitrate/nitrite complexes. The
involvement of a peroxynitrite intermediate in these reactions was implicated. However,
the examples involving the reactions of the cobalt-peroxo complexes of non-porphyrinate
ligands with NO are limited.
5.2 Results and discussion
The addition of 5 equivalents amount of H2O2 in presence of 2 equivalents of NEt3 to the
methanol solution of 2.1 at -40 °C afforded a pink color intermediate which was thermally
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Chapter 5
unstable and decomposed readily at room temperature (Scheme 5.1). In the visible region
of the spectrum, the intermediate displayed a new absorption band at 452 nm (ε/M-1cm-1,
890) (Figure 5.1a). This was attributed to the formation of corresponding Co(III)-peroxo
complex, 5.1.
Scheme 5.1 Reaction of complex 2.1 with H2O2 in methanol at -40 C.
The Nam group has reported a number of transition metal side-on peroxo complexes where
the absorption band in the visible region appeared in the range of 340 nm to 470 nm.23 For
+ + example, the side-on peroxo complexes [(TMC)Co(O2)] , [(15-TMC)Co(O2)] and [(12-
+ TMC)Co(O2)] (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) were
reported to absorb at 436 nm (ε/M-1cm-1, 254), 464 nm (ε/M-1cm-1, 120) and 350 nm (ε/M-
1cm-1, 450), respectively.23,24 On the other hand, the end-on peroxo complexes of Co(III)
displayed absorption bands in the range of 485 to 580 nm.25 The addition of 5 equivalents
of H2O2 and 2 equivalents of NEt3 in methanol solution of 2.1 resulted in the appearance of
a new stretching band at 874 cm-1 in the FT-IR spectrum (Figure 5.1b). The intensity of the
band was found to diminish with time suggesting the unstable nature of the intermediate
(Figure 5.1b). The band appeared to be sensitive to 18O-labelling and shifted to 826 cm-1
18 when H2 O2 was used. Thus, it was assigned to the O-O stretching frequency in the
Co(III)-peroxo intermediate. It is to be noted that in cases of transition metal peroxo
complexes, the O-O stretching frequency which appeared in the range of 830-900 cm-1 are
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TH-1730_126122002
Chapter 5
2 26 designated for side-on (η ) peroxo mode for 1:1 metal/O2 species. In case of structurally
2- + characterized side-on [(TMEN)2Co(O2 )] (TMEN = tetramethylethylenediamine), the O-
O stretching appeared at 861 cm-1 in KBr disc.30b In addition, the O-O stretching in side-on
+ [(Ph2PCH=CHPPh2)2Co(O2)] and [(PhP(CH2CH2CH2PPh2)2Rh]Cl(O2) appeared at 909
-1 -1 27 cm and 862 cm , respectively. Thus, it is logical to believe that addition of H2O2 to the
methanol solution of 2.1 at -40 C resulted in the formation of the corresponding Co(III)-
peroxo intermediate where the peroxo group is bonded to the metal ion in a side-on
fashion. Though, the resonance Raman study of the oxygenated intermediate gives a more
accurate picture of peroxo binding mode in metal-peroxo complexes, the photo-
decomposition of the intermediate in the present case precluded its study.
(a) (b)
Figure 5.1. (a) UV-visible spectra of complex 2.1 (red), after addition of H2O2 {complex 5.1} (grey), in methanol at -40 °C. (b) Solution FT-IR spectra of complexes 2.1 (black), 5.1 (red) in methanol. Green and blue traces represent the gradual decomposition of 5.1 at room temperature.
As expected for a Co(III)-peroxo species, the intermediate was silent in X-band EPR study
(Figure 5.3a).23,24 The ESI-mass spectrum of the intermediate was populated by a peak at
+ m/z 555.28 which is assignable to the mass of [(L1)2Co(O2)] , 5.1 (calculated m/z 555.26)
(Figure 5.3b). The peak was sensitive towards 18O-labelling and found to appear at m/z
18 559.41 when H2 O2 was used (Figures 5.3b and Appendix IV). The upshift of mass by 4
16 18 units upon substitution of O by O suggests the presence of O2 unit in complex 5.1. The
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Chapter 5
observed isotopic distribution pattern was in good agreement with the simulated one
(Appendix IV). Thermal instability of the intermediate precluded its isolation as solid and
further characterization.
(a) (b) Figure 5.3. (a) X-band EPR spectra of complexes 2.1 (black) and 5.1 (red) in methanol at 77 K. 16 (b) ESI-Mass spectra of complex 5.1 obtained from the reaction of 2.1 and H2 O2 in methanol. 18 Inset shows the same when the reaction was carried out with H2 O2.
NO reactivity of the Co(III)-peroxo intermediate
+ The addition of NO gas to the freshly generated complex, [(L1)2Co(O2)] , 5.1 in methanol
at -40 °C followed by warming up at room temperature resulted in the formation of
complex, {[(L1)2Co(NO3)]Cl}, 5.2. It was isolated and characterized by elemental and
spectroscopic analyses (Experimental section and Appendix IV). It was further
characterized by the single crystal X-ray structure determination. The crystallographic
data, important bond angles and distances are listed in tables A5.1, A5.2 and A5.3,
respectively, in the appendix IV. The ORTEP diagram of 5.2 is shown in figure 5.5. The
crystal structure revealed that the Co(II) center is bonded with two units of the ligand and a
- 2 nitrate (NO3 ) anion in a distorted octahedral geometry. An κ -O coordination mode of
- NO3 to the Co(II) center was observed. The Co-Onitrato bond distance is 2.287(4) Å and
Co-O-N bond angle is 94.08 °. The N-O bond distances are 1.273(5) Å and 1.15(1) Å. In
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Chapter 5
the FT-IR spectrum, 5.2 displayed a strong band at 1384 cm-1 which is assignable to the
- 20 - NO3 stretching frequency. It is to be noted that the 5.2 with NO3 as counter anion was
reported earlier by Sun and coworkers.28
Figure 5.5. ORTEP diagram of complex 5.2. (30% thermal ellipsoid plot, H-atoms and solvent molecule are not shown for clarity).
The formation of 5.2 can be envisaged through the formation of a putative peroxynitrite
intermediate (complex 5.1a) in the course of the reaction (Scheme 5.2). It is to be noted
- that NO3 is the common decomposition or isomerization product of the peroxynitrite
anion.8 Since direct spectroscopic evidence of the peroxynitrite intermediate was not
obtained owing to its unstable nature, we sought chemical evidence for the proposed
intermediate. When the reaction of the peroxo complex, 5.1 with NO was followed by the
addition of 2,4-di-tert-butylphenol (DTBP), the exclusive formation of the complex 5.2
was observed and unreacted DTBP (ca. 95%) was recovered. However, when DTBP was
added before the addition of NO (Scheme 5.2), an appreciable nitration (ca. 60%) was
observed. Work-up of the reaction mixture revealed the formation of the corresponding
Co(III)-hydroxo product, 5.3 (ca. 70%) along with 2,4-di-tert-butyl-6-nitrophenol. The
nitration of phenol ring by Co(II)-peroxynitrite intermediate is expected to result in the
formation of corresponding Co(II)-hydroxide (Scheme 5.2). However, only Co(III)-
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Chapter 5
hydroxide was isolated. This is perhaps because of the air sensitivity of the Co(II)-
hydroxide. It has been found in a separate control experiment that in the reaction condition
5.2 does not induce phenol ring nitration and this excludes the possibility of the formation
of nitro-phenol from the reaction of 5.2 with externally added phenol. It is to be noted that
the phenol nitration has been used extensively as an evidence of the formation of metal-
peroxynitrite intermediates.9a,13,15 It would be worth mentioning that even at -80 C
temperature, no indication of formation of a new species was observed in the UV-visible
spectroscopy.
Scheme 5.2. Formation of peroxynitrite intermediate.
5.3 Experimental section
5.3.1 Materials and methods
All reagents and solvents were purchased from commercial sources and were of reagent
grade. HPLC grade methanol was used and dried by heating with iodine-activated
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Chapter 5
magnesium with magnesium loading of ca. 5 g/L. The dried methanol was stored over
20% (m/v) molecular sieves (3 Å) for 3 days before use. Deoxygenation of the solvent and
solutions was effected by repeated vacuum/purge cycles or bubbling with nitrogen or
argon for 30 minutes. NO gas was purified by passing through KOH and P2O5 column. The
dilution of NO was effected with argon gas using Environics Series 4040 computerized gas
dilution system. UV-visible spectra were recorded on an Agilent HP 8454
spectrophotometer. FT-IR spectra were taken on a Perkin-Elmer spectrophotometer with
either sample prepared as KBr pellets or in solution in a KBr cell. 1H-NMR spectra were
obtained with a 400 MHz Varian FT spectrometer. Chemical shifts (ppm) were referenced
either with an internal standard (Me4Si) for organic compounds or to the residual solvent
peaks. The X-band Electron Paramagnetic Resonance (EPR) spectra were recorded on a
JES-FA200 ESR spectrometer, at room temperature or at 77 K. Spectra were recorded
with all the samples prepared in methanol solution at 77 K. The experimental conditions
[Frequency, 9.136 GHz; Power, 0.995 mW; Field center, 336.00 mT, Width, 250.00 mT;
Sweep time, 30.0 s; Modulation frequency, 100.00 kHz, Width, 1.000 mT; Amplitude, 1.0
and Time constant, 0.03 s] kept same for all the samples. Mass spectra of the compounds
were recorded in a Waters Q-Tof Premier and Aquity instrument. Solution electrical
conductivity was checked using a Systronic 305 conductivity bridge. The magnetic
moment of complexes were measured on a Cambridge Magnetic Balance. Elemental
analyses were obtained from a Perkin Elmer Series II Analyzer.
Single crystals of 5.2 were grown by slow evaporation technique from methanol. The
intensity data were collected using a Bruker SMART APEX-II CCD difractometer,
equipped with a fine focus 1.75 kW sealed tube Mo Kα radiation (λ = 0.71073 Å) at 273(3)
K, with increasing ω (width of 0.3° per frame) at a scan speed of 3 s/frame. The SMART
software was used for data acquisition. 29 Data integration and reduction was undertaken
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with SAINT and XPREP software. Structures were solved by direct methods using
SHELXS-97 and refined with full-matrix least squares on F2 using SHELXL-97. 30
Structural illustrations have been drawn with ORTEP-3 for Windows. 31
5.3.2 Syntheses of complexes
+ (a) 5.1, [(L1)2Co(O2)]
Complex 2.1 (1.18 g, 2 mmol) was dissolved in 20 mL of dried and degassed methanol in a
50 mL Schlenk fask fitted with a rubber septum and connected to the Schlenk line. The
solution was cooled at -40 °C. To this cold solution, pre-cooled 5 equivalents of hydrogen
peroxide (37% v/v, 0.8 mL) and 2 equivalents amount of NEt3 were added and the solution
turned pink. It was then stirred for 10 minutes at -40 C to afford complex 5.1. The
complex was found to be thermally unstable. So attempts were not made to isolate it. All
the spectral characterization have been done using freshly prepared complex 5.1 in
-1 -1 -1 -1 methanol at -40 C. UV-visible (CH3OH): 452 nm (ε/M cm , 890), 559 nm (ε/M cm ,
-1 419). FT-IR (CH3OH/KBr): 1637, 1054, 874, 817 cm . ESI-Mass (m/z): Calcd: 555.28,
found: 555.26 (molecular ion peak).
+ (b) 5.2, [(L1)2Co(NO3)]
The peroxo complex 5.1 was prepared freshly in a 50 mL Schlenk flask under Ar
atmosphere as stated above. To this freshly generated 5.1 in methanol at -40 C, NO gas
was added through a gas-tight syringe fitted in the Schlenk line. The resulting solution was
stirred for 10 min; then warmed up to room temperature and the excess of NO gas was
removed by applying several cycles of vacuum and Ar purge. Then the solution was kept
for slow evaporation in air. After a few days, crystals of 5.2 were obtained. Elemental
analyses for C26H40ClCoN9O3, Calcd. (%): C, 50.28; H, 6.43; N, 20.29, found (%): C,
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-1 -1 50.40; H, 6.44; N, 20.41. UV-visible (CH3OH): 532 nm (ε/M cm , 190). FT-IR (in KBr):
-1 2 -1 2975, 1634, 1580, 1470, 1384, 1319, 1086 cm . Conductivity (CH3OH): 124 Scm mol .
X-band EPR: gav, 2.16. Observed magnetic moment: 1.81 µB.
(d) 5.3 and 2,4-di-tert-butyl-6-nitrophenol
Complex 2.1 (1.18 g, 2 mmol) was dissolved in 20 mL of dried and degassed methanol
under Ar atmosphere in a 50 mL Schlenk fask with a rubber septum and fitted in the
Schlenk line and the solution was cooled at -40 °C. To this cold solution, pre-cooled
hydrogen peroxide (37% v/v, 0.8 mL, 5 equivalent) was added and stirred for 10 min at -40
C to result in the formation of 5.1. To this solution, the NO gas was added through a gas-
tight syringe fitted in the Schlenk line followed by a methanol (5 mL) solution containing
2,4-di-tert-butylphenol (2.06 g; 10 mmol) pre-cooled at -40 C and the mixture was stirred
for 30 min at -40 °C. The reaction mixture was then warmed up to room temperature and
the excess of NO gas was removed by applying several cycles of vacuum and Ar purge.
Then the solution was dried using rotavapor. The solid mass was then subjected to column
chromatography using silica gel column. Pure 2, 4-di-tert-butyl-6-nitrophenol {Yield:
0.305 g (60 %)} was eluted with 100% hexane and complex 5.3 was eluted with methanol.
Complex 5.3: Yield: 0.800 g (65 %). Elemental analyses for C26H41Cl2CoN8O, Calcd. (%):
C, 51.07; H, 6.76; N, 18.32, found (%): C, 51.01; H, 6.80; N, 18.38. UV-visible (MeOH):
507 nm (ε/M-1cm-1, 67), 565 nm (ε/M-1cm-1, 107) and 615 nm (ε/M-1 cm-1, 90). FT-IR (in
KBr): 3450, 2977, 1636, 1452, 1078, 920, 838 cm-1.
5.4 Conclusion
In conclusion, the present study demonstrate an example of a Co(II) complex, 2.1 which in
methanol solution reacted with H2O2 at -40 C to result in the corresponding Co(III)-
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peroxo complex, 5.1. It was characterized by spectroscopic analyses. The peroxo
intermediate, 5.1 upon reaction with NO, resulted a Co(II)-nitrato complex, 5.2
presumably through the formation of Co(II)-peroxynitrite intermediate. It was evidenced
by the external phenol ring nitration reaction.
5.5 References
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Tammanstrasse 4, D-3400 Gottingen, Germany 1999. (b) Sheldrick, G. M.
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TH-1730_126122002 Appendix I
Figure A1.1. FT-IR spectrum of L1 in KBr pellet.
1 Figure A1.2. H-NMR spectrum of L1 in CD3OD.
13 Figure A1.3. C- NMR spectrum of L1 in CD3OD.
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Figure A1.4. FT- IR spectrum of complex 2.1 in KBr pellet.
Figure A1.5. UV-visible spectrum of complex 2.1 in methanol.
1 Figure A1.6. H-NMR spectrum of complex 2.3 in CD3OD.
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Figure A1.7. ESI Mass spectrum of complex 2.1 in methanol. Inset shows the (a) experimental and (b) simulated isotopic distribution pattern.
Figure A1.8. X-band EPR spetcrum of complex 2.1 in methanol at 77 K.
Figure A1.9. FT-IR spetcrum of complex 2.3 in KBr.
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Figure A1.10. UV-visible spetcrum of complex 2.3 in methanol.
Figure A1.11. ESI Mass spectrum of complex 2.3 in methanol.
1 Figure A1.12. H-NMR spectrum of complex 2.3 in CD3OD.
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Table A1.1. Crystallographic data for complexes 2.1 and 2.3.
2.1 2.3
Formulae C52 H90 Cl4 Co2 N16 O5 C26 H38ClCoN9O4 Mol. wt. 1279.06 635.03 Crystal system Monoclinic Monoclinic Space group P21/n P 2/c Temperature /K 293(2) 296(2) Wavelength /Å 0.71073 0.71073 a /Å 14.2005(9) 9.4404(6) b /Å 14.2147(10) 9.3883(9) c /Å 32.3238(18) 18.5255(11) α/° 90.00 90.00 β/° 96.555(6) 91.110(6) γ/° 90.00 90.00 V/ Å3 6482.1(7) 1641.6(2) Z 4 2 Density/Mgm-3 1.311 1.285 Abs. Coeff. /mm-1 0.732 0.648 Abs. correction Multi-scan Multi-scan F(000) 2704 666 Total no. of reflections 11364 2886 Reflections, I > 2σ(I) 7638 2474 Max. 2θ/° 25.00 25.00 Ranges (h, k, l) -16≤ h ≤16 -11≤ h ≤11 -16≤ k ≤12 -11≤ k ≤5 -27≤ l ≤ 38 -22≤ l ≤ 13 Complete to 2θ (%) 99.7 99.8 Refinement method Full-matrix least-squares Full-matrix least-squares on F2 on F2 Goof (F2) 1.163 0.989 R indices [I > 2σ(I)] 0.086 0.0674 R indices (all data) 0.1190 0.0755
Table A1.2. Selected bond lengths (Å) for complexes 2.1 and 2.3.
Atoms 2.1 2.3 Co(1)-N(1) 1.999(5) 2.100(3) Co(1)-N(3) 1.972(5) 2.100(3) N(1)-C(3) 1.330(9) 1.340(5) N(1)-C(6) 1.389(7) 1.400(4) N(2)-C(3) 1.322(8) 1.3450(5) N(2)-C(4) 1.368(8) 1.390(5) N(3)-C(8) 1.399(7) 1.390(4)
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N(3)-C(11) 1.326(9) 1.330(5) Co(1)-O(1) - 2.230(3) O(1)-N(5) - 1.29(4)
Table A1.3. Selected bond angles (°) for complexes 2.1 and 2.3.
Atoms 2.1 2.3 N(3)-Co(1)-N(1) 95.8(2) 89.0(1) Co(1)-N(1)-C(3) 128.0(4) 131.0(2) Co(1)-N(1)-C(6) 123.4(4) 121.0(2) Co(1)-N(3)-C(8) 125.2(4) 123.0(2) Co(1)-N(3)-C(11) 127.8(4) 131.0(2) C(3)-N(2)-C(4) 109.6(5) 109.0(3) C(8)-N(3)-C(11) 106.7(5) 106.0(3) N(1)-C(3)-C(2) 126.4(6) 126.0(3) N(2)-C(4)-C(6) 105.9(5) 106.0(3) N(1)-C(3)-N(2) 109.3(5) 110.0(3) N(3)-Co(1)-O(1) - 85.0(1) O(1)-N(5)-O(1) - 108.0(4) Co(1)-N(5)-O(1) - 98.0(3)
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Figure A2.1. FT-IR spectrum of ligand L2H2 in KBr.
1 Figure A2.2. H-NMR spectrum of ligand L2H2 in CDCl3.
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13 Figure A2.3. C-NMR spectrum of ligand L2H2 in CDCl3.
Figure A2.4. FT-IR spectrum of complex 3.1 in KBr.
Figure A2.5. UV-visible spectrum of complex 3.1 in dry THF.
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Figure A2.6. X-band EPR spectra of complexes 3.1 (black), 3.2 (red) and 3.3 (green) in dry THF at 77 K.
Figure A2.7. FT-IR spectrum of complex 3.2 in KBr.
Figure A2.8. UV-visible spectrum of complex 3.2 in dry THF.
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Figure A2.9. UV-visible spectrum of complex 3.3 in dry THF.
Figure A2.10. ESI-mass spectrum of complex 3.3 with isotopic distribution (a) experimental (b) + simulated in methanol [(L2)Co(NO3) + Na] .
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Figure A2.11. FT-IR spectrum of ligand L3H2 in KBr.
1 Figure A2.12. H-NMR spectrum of ligand L3H2 in CDCl3.
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Figure A2.13. ESI-mass spectrum of ligand L3H2 in acetonitrile.
Figure A2.14. FT-IR spectrum of complex 3.4 in KBr.
Figure A2.15. UV-visible spectrum of complex 3.4 in dry THF.
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Figure A2.16. ESI-mass spectrum of complex 3.4 with isotopic distribution (a) experimental (b) simulated in methanol.
Figure A2.17. UV-visible spectrum of complex 3.5 in dry THF.
Figure A2.18. FT-IR spectrum of complex 3.4 in KBr.
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1 Figure A2.19. H-NMR spectrum of complex 3.5 in CDCl3.
1 Figure A2.20. H-NMR spectrum of 2,4-di-tert-butyl-6-nitrophenol in CDCl3.
13 Figure A2.21. C-NMR spectrum of 2,4-di-tert-butyl-6-nitrophenol in CDCl3.
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Figure A2.22. ESI-mass spectrum of 2,4-di-tert-butyl-6-nitrophenol in methanol.
Table A2.1. Crystallographic data for complexes 3.1, 3.2, 3.4 and 3.5.
3.1 3.2 3.4 3.5
Formulae C32H46CoN2O2 C32H46CoN3O3 C56H48Br4CoN4O3 C48H32Br4Co N5O2 Mol. wt. 549.64 579.65 1203.55 1089.32 Crystal system Orthorhombic Monoclinic Monoclinic Monoclinic Space group P b c n P2(1)/c C 1 2/c 1 Cc Temperature /K 296(2) 296(2) 296(2) 296(2) Wavelength /Å 0.71073 0.71073 0.71073 0.71073 a /Å 26.7490(14) 15.141(3) 24.765(3) 32.6508(19) b /Å 11.0089(4) 19.113(3) 9.5985(5) 9.0163(7) c /Å 10.3143(5) 11.689(2) 24.578(4) 15.6113(9) α/° 90.00 90.00 90.00 90.00 β/° 90.0 105.886(6) 120.213(16) 114.564(5) γ/° 90.00 90.00 90.00 90.00 V/ Å3 3037.3(2) 3253.4(10) 5048.8(9) 4179.9(5) Z 8 4 4 4 Density/Mgm-3 1.2020 1.183 1.583 1.928 Abs. Coeff. /mm-1 0.594 0.561 3.554 6.390 Abs. correction Multi-scan None Multi-scan Multi-scan F(000) 1180.0 1240 2412 2152.0 Total no. of 2037 48481 3375 5886 reflections Reflections,I> 2σ(I) 3525 5610 2610 4379 Max. 2θ/° 26.32 25.00 24.50 25.25 Ranges (h, k, l) -35≤ h ≤31 -18≤ h ≤16 -28≤ h ≤26 -39≤ h ≤38 -14≤ k ≤14 -22≤ k ≤22 -11≤ k ≤7 -10≤ k ≤8 -13≤ l ≤ 13 -13≤ l ≤ 13 -25≤ l ≤ 19 -18≤ l ≤ 18 Complete to 2θ (%) 99.66 98.1 80.1 99.8 Goof (F2) 0.792277 1.045 0.955 1.069
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R indices[I > 2σ(I)] 0.049337 0.0550 0.0604 0.0515 R indices (all data) 0.082334 0.0903 0.0813 0.0755
Table A2.2. Selected bond lengths (Å) for complexes 3.1, 3.2, 3.4 and 3.5.
Atoms 3.1 3.2 3.4 3.5 Co(1)-N(1) 1.848(2) 1.880(3) 1.992(6) 1.97(1) Co(1)-N(3) - 1.799(4) - 1.98(1) Co(1)-O(1) 1.845(2) 1.904(2) 2.382(7) O(1)-C(1) 1.315(3) 1.317(3) - N(3)-O(3) 1.155(6) - 1.330(6) Co(1)-N(5) - - 1.853(6) N(1)-C(15) 1.293(4) 1.295(5) - N(5)-O(1) - - 1.19(2)
Table A2.3. Selected bond angles (°) for complexes 3.1, 3.2, 3.4 and 3.5.
Atoms 3.1 3.2 3.4 3.5 O(1)-Co(1)-N(1) 93.38(9) 92.7(1) 92.8(2) - O(1)-Co(1)-N(2) - 167.8(1) 88.6(2) - N(3)-Co(1)-N(1) - 99.9(2) - 172.0(5) Co(1)-N(1)-C(1) 112.7(2) - 128.1(5) 128.7(9) N(1)-Co(1)-N(2) - - 89.6(2) 89.7(5) Co(1)-N(5)-O(1) - - - 119(1) Co(1)-N(3)-O(3) - 126.5(4) - 121.9(3) C(1)-O(1)-Co(1) 129.23(17) 127.5(2) - -
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Figure A3.1. FT-IR spectrum of ligand L4H2 in KBr.
1 Figure A3.2. H-NMR spectrum of ligand L4H2 in CDCl3.
Figure A3.3. ESI mass spectrum of L4H2 in acetonitrile.
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Figure A3.4. ORTEP diagram of ligand L4H2 (30% thermal ellipsoid plot; H-atoms and solvent molecules are omitted for clarity).
Figure A3.5. UV-visible spectrum of complex 4.1 in dichloromethane.
Figure A3.6. FT-IR spectrum of complex 4.1 in KBr.
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1 Figure A3.7. H-NMR spectrum of complex 4.1 in CDCl3.
Figure A3.8. ESI-mass spectrum of complex 4.1 in acetonitrile. Inset: (a) experimental and (b) simulated isotopic distribution pattern.
Figure A3.9. UV-visible spectrum of complex 4.2 in dichloromethane.
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1 Figure A3.10. H-NMR spectrum of complex 4.2 in CDCl3.
Figure A3.11. UV-visible spectrum of complex 4.3 in acetonitrile.
Figure A3.12. FT-IR spectrum of complex 4.3 in KBr.
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1 Figure A3.13. H-NMR spectrum of complex 4.3 in CDCl3.
Figure A3.14. FT-IR spectrum of complex 4.4 in KBr.
Figure A3.15. UV-visible spectrum of complex 4.4 in acetonitrile.
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Table A3.1. Crystallographic data of ligand L4H2, complexes 4.1 and 4.2.
L4H2 4.1 4.2 Formulae C44 H26 Cl4 N4 O3 C56 H48 Cl4 N4 Co C48 H32 Cl4 N5 Co O2 O3 Mol. wt. 800.49 1025.71 911.52 Crystal system Triclinic Monoclinic Monoclinic Space group P -1 C 1 2/c 1 P 121/c1 Temperature /K 296(2) 296(2) 100(2) Wavelength /Å 0.71073 0.71073 0.71073 a /Å 10.0674(5) 24.482(3) 15.2052(14) b /Å 13.1785(11) 9.5254(6) 15.4115(12) c /Å 15.4982(12) 24.225(3) 17.6398(16) α/° 85.437(7) 90.00 90.00 β/° 80.873(5) 119.561(14) 103.442(10) γ/° 88.471(5) 90.00 90.00 V/ Å3 2023.5(3) 4913.8(8) 4020.4(6) Z 2 4 4 Density/Mgm-3 1.314 1.386 1.506 Abs. Coeff. /mm-1 0.337 0.616 0.742 Abs. correction Multi-scan Multi-scan Multi-scan F(000) 820 2124 1864 Total no. of 7104 4320 7257 reflections Reflections, I > 4624 3541 4976 2σ(I) Max. 2θ/° 25.00 25.00 25.25 Ranges (h, k, l) -11≤ h ≤11 -25≤ h ≤29 -18≤ h ≤9 -15≤ k ≤13 -11≤ k ≤10 -18≤ k ≤11 -18≤ l ≤ 18 -26≤ l ≤ 28 -21≤ l ≤ 21 Complete to 2θ (%) 99.8 99.8 99.7 Refinement method Full-matrix least- Full-matrix least- Full-matrix least- squares on F2 squares on F2 squares on F2 Goof (F2) 1.014 0.802 0.0992 R indices [I > 2σ(I)] 0.1096 0.0471 0.0695 R indices (all data) 0.1438 0.0587 0.1321
Table A3.2. Selected bond lengths (Å) of ligand L4H2, complexes 4.1 and 4.2.
Atoms L4H2 4.1 4.2 Co(1)–N(1) - 1.991(2) 1.974(6) Co(1)–N(2) - 1.987(2) 1.990(6) Co(1)–N(3) - - 1.979(6) Co(1)–N(4) - - 1.982(6) Co(1)–N(5) - - 1.846(7) C(1)-N(1) 1.373(9) 1.381(4) 1.380(1) C(1)–C(2) 1.410(1) 1.393(4) 1.40(1) C(3)-C(4) 1.390(1) 1.385(4) 1.398(9)
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C(4)-C(5) 1.370(1) 1.395(5) 1.380(1) C(5)-C(6) 1.380(1) 1.373(6) 1.390(1) C(6)-Cl(1) 1.749(8) 1.751(4) 1.747(7) N(5)–O(1) - - 1.230(1)
Table A3.3. Selected bond angles (˚) of ligand L4H2, complexes 4.1 and 4.2.
Atoms L4H2 4.1 4.2 N1–Co1–N2 - 89.9(9) 89.4(2) N1–Co1–N3 - - 171.6(2) N2–Co1–N3 - - 89.9(2) N3–Co1–N4 - - 89.7(2) N2–Co1–N4 - - 170.2(2) N1–Co1–N5 - - 94.2(3) N2–Co1–N5 - - 92.8(3) Co1-N5-O1 - - 118.4(6) N1-C1-C2 125.9(7) 125.8(3) 125.4(7) C1-C2-C3 116.5(6) 117.8(3) 116.5(6)
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TH-1730_126122002 Appendix IV
Figure A4.1.UV-visible spectrum of complex 5.1 in methanol at -40 °C.
Figure A4.2. FT-IR spectrum of complex 5.1 in KBr.
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Appendix IV
Figure A4.3. ESI-mass spectrum of complex 5.1 in methanol. Inset: (a) experimental and (b) simulated isotopic distribution pattern.
Figure A4.4. ESI-Mass spectrum of complex 5.1 obtained from the reaction of 2.1 and 18 H2 O2 in methanol.
Figure A4.5. FT-IR spectrum of complex 5.2 in KBr.
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Figure A4.6. UV-visible spectrum of complex 5.2 in methanol.
Figure A4.7. X-Band EPR spectrum of complex 5.2 in methanol at room temperature.
Figure A4.8. ESI Mass spectrum of complex 5.2 in methanol. The m/z, 523.23 peak
corresponds to the {(L1)2Co} unit.
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Figure A4.9. FT-IR spectrum of complex 5.3 in KBr pellet.
Figure A4.10. UV-visible spectrum of complex 5.3 in methanol.
Table A4.1. Crystallographic data for complex 5.2.
5.2
Formulae C26 H40Cl N9 Co O5 Mol. wt. 653.05 Crystal system Monoclinic Space group P 2/c Temperature /K 293(2) Wavelength /Å 0.71073 a /Å 9.4505(3) b /Å 9.5348(3) c /Å 18.4708(5) α/° 90.00 β/° 91.015(2)
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γ/° 90.00 V/ Å3 1664.12(9) Z 2 Density/Mgm-3 1.310 Abs. Coeff. /mm-1 0.643 Abs. correction None F(000) 686 Total no. of reflections 2913 Reflections, I > 2σ(I) 2303 Max. 2θ/° 24.99 Ranges (h, k, l) -11≤ h ≤11 -11≤ k ≤11 -21≤ l ≤ 21 Complete to 2θ (%) 99.4 Refinement method Full-matrix least-squares on F2 Goof (F2) 0.910 R indices [I > 2σ(I)] 0.0627 R indices (all data) 0.0763
Table A4.2. Selected bond lengths (Å) for complex 5.2.
Atoms 5.2 Co(1)-N(1) 2.087(3) Co(1)-N(3) 2.099(3) Co(1)-O(1) 2.287(4) O(1)-N(5) 1.273(5) O(2)-N(5) 1.15(1)
Table A4.3. Selected bond angles (°) for complex 5.2.
Atoms 5.2 N(3)-Co(1)-N(1) 98.2(1) O(1)-Co(1)-N(1) 84.5(1) O(1)-Co(1)-N(3) 103.6(2) Co(1)-O(1)-N(5) 93.1(4) O(2)-N(5)-O(1) 121.3(9)
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TH-1730_126122002 List of Publications
(1) “Reductive nitrosylation of nickel(II) complex by nitric oxide followed by nitrous
oxide release”.
Ghosh, S.; Deka, H.; Dangat, Y. B.; Saha, S.; Gogoi, K.; Vanka, K.; Mondal, B.
Dalton Trans. 2016, 45, 10200.
(2) “Effect of ligand denticity on the nitric oxide reactivity of cobalt(II) complexes” .
Deka, H.; Ghosh, S.; Saha, S.; Gogoi, K.; Mondal, B. Dalton Trans. 2016, 45,
10979.
(3) “Nitrogen dioxide reactivity of a nickel(II) complex of tetraazacyclotetradecane
ligand”.
Ghosh, S.; Deka, H.; Saha, S.; Mondal, B. Inorg. Chim. Acta 2017, 466, 285.
(4) “Nitric oxide reactivity of a Cu(II) complex of an imidazole based ligand: Aromatic
C-nitrosation followed by the formation of N-nitrosohydroxylaminato complex”.
Deka, H.; Ghosh, S.; Gogoi, K.; Saha, S.; Mondal, B. Inorg. Chem. 2017, 56, 5034.
(5) “Reaction of a nitrosyl complex of cobalt-porphyrin with hydrogen peroxide:
Putative formation of peroxynitrite intermediate”.
Saha, S.; Gogoi, K.; Mondal, B.; Ghosh, S.; Deka, H.; Mondal, B. Inorg. Chem.
2017, 56, 7781.
(6) “Reaction of a Co(III)-peroxo complex and NO: Formation of a putative
peroxynitrite intermediate”.
Saha, S.; Ghosh, S.; Gogoi, K.; Deka, H.; Mondal, B.; Mondal, B. Inorg. Chem.
2017, 56, 10932.
(7) “Dioxygenation reaction of a cobalt-nitrosyl: Putative formation of a cobalt–
III - peroxynitrite via a {Co (NO)(O2 )} intermediate”.
TH-1730_126122002
Gogoi, K.; Saha, S.; Ghosh, S.; Deka, H.; Mondal, B.; Mondal, B. Inorg. Chem.
2017, 56, 14438.
(8) “Nitric oxide reactivity of Cu(II) complex of N- donor ligand: Formation of a stable
nitrous oxide complex”.
Deka, H.; Dangat, Y. B.; Saha, S.; Gogoi, K.; Vanka, K.; Mondal, B. (Under
revision).
(9) “Reductive nitrosylation of cobalt(II) complex by nitric oxide followed by release
of nitrous oxide”.
Saha, S.; Gogoi, K.; Mondal, B. (Communicated)
(10) “Reactivity of a cobalt nitrosyl complex: Formation of a cobalt peroxynitrite
intermediate and transfer the nitrosyl to a cobalt porphyrin complex”.
Saha, S.; Gogoi, K.; Mondal, B. (Communicated)
7 9 (11) “Disproportionation of a {FeNO} species into {Fe(NO)2} and ferric complex”.
Gogoi, K.; Boro, M.; Saha, S.; Mondal, B. (Communicated).
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